diff --git a/src/Facebook/Authentication/OAuth2Client.php b/src/Facebook/Authentication/OAuth2Client.php
index 5016465c6..455c451ba 100644
--- a/src/Facebook/Authentication/OAuth2Client.php
+++ b/src/Facebook/Authentication/OAuth2Client.php
@@ -74,13 +74,13 @@ class OAuth2Client
     /**
      * @param FacebookApp    $app
      * @param FacebookClient $client
-     * @param string|null    $graphVersion The version of the Graph API to use.
+     * @param string         $graphVersion The version of the Graph API to use.
      */
-    public function __construct(FacebookApp $app, FacebookClient $client, $graphVersion = null)
+    public function __construct(FacebookApp $app, FacebookClient $client, $graphVersion)
     {
         $this->app = $app;
         $this->client = $client;
-        $this->graphVersion = $graphVersion ?: Facebook::DEFAULT_GRAPH_VERSION;
+        $this->graphVersion = $graphVersion;
     }
 
     /**
diff --git a/src/Facebook/Facebook.php b/src/Facebook/Facebook.php
index 7e40c52f9..7ac784cd5 100644
--- a/src/Facebook/Facebook.php
+++ b/src/Facebook/Facebook.php
@@ -53,11 +53,6 @@ class Facebook
      */
     const VERSION = '6.0-dev';
 
-    /**
-     * @const string Default Graph API version for requests.
-     */
-    const DEFAULT_GRAPH_VERSION = 'v2.7';
-
     /**
      * @const string The name of the environment variable that contains the app ID.
      */
@@ -120,7 +115,7 @@ public function __construct(array $config = [])
         $config = array_merge([
             'app_id' => getenv(static::APP_ID_ENV_NAME),
             'app_secret' => getenv(static::APP_SECRET_ENV_NAME),
-            'default_graph_version' => static::DEFAULT_GRAPH_VERSION,
+            'default_graph_version' => null,
             'enable_beta_mode' => false,
             'http_client_handler' => null,
             'persistent_data_handler' => null,
@@ -133,6 +128,9 @@ public function __construct(array $config = [])
         if (!$config['app_secret']) {
             throw new FacebookSDKException('Required "app_secret" key not supplied in config and could not find fallback environment variable "' . static::APP_SECRET_ENV_NAME . '"');
         }
+        if (!$config['default_graph_version']) {
+            throw new \InvalidArgumentException('Required "default_graph_version" key not supplied in config');
+        }
 
         $this->app = new FacebookApp($config['app_id'], $config['app_secret']);
         $this->client = new FacebookClient(
@@ -148,7 +146,6 @@ public function __construct(array $config = [])
             $this->setDefaultAccessToken($config['default_access_token']);
         }
 
-        // @todo v6: Throw an InvalidArgumentException if "default_graph_version" is not set
         $this->defaultGraphVersion = $config['default_graph_version'];
     }
 
diff --git a/src/Facebook/FacebookRequest.php b/src/Facebook/FacebookRequest.php
index 2edd103a9..a03127442 100644
--- a/src/Facebook/FacebookRequest.php
+++ b/src/Facebook/FacebookRequest.php
@@ -102,7 +102,7 @@ public function __construct(FacebookApp $app = null, $accessToken = null, $metho
         $this->setEndpoint($endpoint);
         $this->setParams($params);
         $this->setETag($eTag);
-        $this->graphVersion = $graphVersion ?: Facebook::DEFAULT_GRAPH_VERSION;
+        $this->graphVersion = $graphVersion;
     }
 
     /**
@@ -490,7 +490,7 @@ public function getPostParams()
     /**
      * The graph version used for this request.
      *
-     * @return string
+     * @return string|null
      */
     public function getGraphVersion()
     {
diff --git a/src/Facebook/Helpers/FacebookPageTabHelper.php b/src/Facebook/Helpers/FacebookPageTabHelper.php
index 8e7be7b20..9b419644d 100644
--- a/src/Facebook/Helpers/FacebookPageTabHelper.php
+++ b/src/Facebook/Helpers/FacebookPageTabHelper.php
@@ -43,9 +43,9 @@ class FacebookPageTabHelper extends FacebookCanvasHelper
      *
      * @param FacebookApp    $app          The FacebookApp entity.
      * @param FacebookClient $client       The client to make HTTP requests.
-     * @param string|null    $graphVersion The version of Graph to use.
+     * @param string         $graphVersion The version of Graph to use.
      */
-    public function __construct(FacebookApp $app, FacebookClient $client, $graphVersion = null)
+    public function __construct(FacebookApp $app, FacebookClient $client, $graphVersion)
     {
         parent::__construct($app, $client, $graphVersion);
 
diff --git a/src/Facebook/Helpers/FacebookSignedRequestFromInputHelper.php b/src/Facebook/Helpers/FacebookSignedRequestFromInputHelper.php
index a0cff3dd8..5d213795f 100644
--- a/src/Facebook/Helpers/FacebookSignedRequestFromInputHelper.php
+++ b/src/Facebook/Helpers/FacebookSignedRequestFromInputHelper.php
@@ -57,12 +57,11 @@ abstract class FacebookSignedRequestFromInputHelper
      *
      * @param FacebookApp    $app          The FacebookApp entity.
      * @param FacebookClient $client       The client to make HTTP requests.
-     * @param string|null    $graphVersion The version of Graph to use.
+     * @param string         $graphVersion The version of Graph to use.
      */
-    public function __construct(FacebookApp $app, FacebookClient $client, $graphVersion = null)
+    public function __construct(FacebookApp $app, FacebookClient $client, $graphVersion)
     {
         $this->app = $app;
-        $graphVersion = $graphVersion ?: Facebook::DEFAULT_GRAPH_VERSION;
         $this->oAuth2Client = new OAuth2Client($this->app, $client, $graphVersion);
 
         $this->instantiateSignedRequest();
diff --git a/src/Facebook/Url/FacebookUrlManipulator.php b/src/Facebook/Url/FacebookUrlManipulator.php
index 870071091..065dd94d1 100644
--- a/src/Facebook/Url/FacebookUrlManipulator.php
+++ b/src/Facebook/Url/FacebookUrlManipulator.php
@@ -142,12 +142,12 @@ public static function mergeUrlParams($urlToStealFrom, $urlToAddTo)
      *
      * @param string|null $string
      *
-     * @return string|null
+     * @return string
      */
     public static function forceSlashPrefix($string)
     {
         if (!$string) {
-            return $string;
+            return '';
         }
 
         return strpos($string, '/') === 0 ? $string : '/' . $string;
diff --git a/tests/FacebookBatchRequestTest.php b/tests/FacebookBatchRequestTest.php
index 4d41a6f4d..d2079722a 100755
--- a/tests/FacebookBatchRequestTest.php
+++ b/tests/FacebookBatchRequestTest.php
@@ -23,7 +23,6 @@
  */
 namespace Facebook\Tests;
 
-use Facebook\Facebook;
 use Facebook\FacebookApp;
 use Facebook\FacebookRequest;
 use Facebook\FacebookBatchRequest;
@@ -216,7 +215,6 @@ public function testBatchRequestEntitiesProperlyGetConvertedToAnArray($request,
     public function requestsAndExpectedResponsesProvider()
     {
         $headers = $this->defaultHeaders();
-        $apiVersion = Facebook::DEFAULT_GRAPH_VERSION;
 
         return [
             [
@@ -224,7 +222,7 @@ public function requestsAndExpectedResponsesProvider()
                 [
                     'headers' => $headers,
                     'method' => 'GET',
-                    'relative_url' => '/' . $apiVersion . '/foo?foo=bar&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9',
+                    'relative_url' => '/foo?foo=bar&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9',
                     'name' => 'foo_name',
                 ],
             ],
@@ -233,7 +231,7 @@ public function requestsAndExpectedResponsesProvider()
                 [
                     'headers' => $headers,
                     'method' => 'POST',
-                    'relative_url' => '/' . $apiVersion . '/bar',
+                    'relative_url' => '/bar',
                     'body' => 'bar=baz&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9',
                     'name' => 'foo_name',
                 ],
@@ -243,7 +241,7 @@ public function requestsAndExpectedResponsesProvider()
                 [
                     'headers' => $headers,
                     'method' => 'DELETE',
-                    'relative_url' => '/' . $apiVersion . '/bar?access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9',
+                    'relative_url' => '/bar?access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9',
                     'name' => 'foo_name',
                 ],
             ],
@@ -273,7 +271,7 @@ public function testBatchRequestsWithFilesGetConvertedToAnArray()
         $this->assertEquals([
             'headers' => $this->defaultHeaders(),
             'method' => 'POST',
-            'relative_url' => '/' . Facebook::DEFAULT_GRAPH_VERSION . '/bar',
+            'relative_url' => '/bar',
             'body' => 'message=foobar&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9',
             'name' => 'foo_name',
             'attached_files' => $attachedFiles,
@@ -290,10 +288,9 @@ public function testPreppingABatchRequestProperlySetsThePostParams()
         $params = $batchRequest->getParams();
 
         $expectedHeaders = json_encode($this->defaultHeaders());
-        $version = Facebook::DEFAULT_GRAPH_VERSION;
         $expectedBatchParams = [
-            'batch' => '[{"headers":' . $expectedHeaders . ',"method":"GET","relative_url":"\\/' . $version . '\\/foo?access_token=bar_token&appsecret_proof=2ceec40b7b9fd7d38fff1767b766bcc6b1f9feb378febac4612c156e6a8354bd","name":"foo_name"},'
-                . '{"headers":' . $expectedHeaders . ',"method":"POST","relative_url":"\\/' . $version . '\\/bar","body":"foo=bar&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9"}]',
+            'batch' => '[{"headers":' . $expectedHeaders . ',"method":"GET","relative_url":"\\/foo?access_token=bar_token&appsecret_proof=2ceec40b7b9fd7d38fff1767b766bcc6b1f9feb378febac4612c156e6a8354bd","name":"foo_name"},'
+                . '{"headers":' . $expectedHeaders . ',"method":"POST","relative_url":"\\/bar","body":"foo=bar&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9"}]',
             'include_headers' => true,
             'access_token' => 'foo_token',
             'appsecret_proof' => 'df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9',
@@ -317,10 +314,9 @@ public function testPreppingABatchRequestProperlyMovesTheFiles()
         $attachedFiles = implode(',', array_keys($files));
 
         $expectedHeaders = json_encode($this->defaultHeaders());
-        $version = Facebook::DEFAULT_GRAPH_VERSION;
         $expectedBatchParams = [
-            'batch' => '[{"headers":' . $expectedHeaders . ',"method":"GET","relative_url":"\\/' . $version . '\\/foo?access_token=bar_token&appsecret_proof=2ceec40b7b9fd7d38fff1767b766bcc6b1f9feb378febac4612c156e6a8354bd","name":"foo_name"},'
-                . '{"headers":' . $expectedHeaders . ',"method":"POST","relative_url":"\\/' . $version . '\\/me\\/photos","body":"message=foobar&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9","attached_files":"' . $attachedFiles . '"}]',
+            'batch' => '[{"headers":' . $expectedHeaders . ',"method":"GET","relative_url":"\\/foo?access_token=bar_token&appsecret_proof=2ceec40b7b9fd7d38fff1767b766bcc6b1f9feb378febac4612c156e6a8354bd","name":"foo_name"},'
+                . '{"headers":' . $expectedHeaders . ',"method":"POST","relative_url":"\\/me\\/photos","body":"message=foobar&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9","attached_files":"' . $attachedFiles . '"}]',
             'include_headers' => true,
             'access_token' => 'foo_token',
             'appsecret_proof' => 'df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9',
diff --git a/tests/FacebookClientTest.php b/tests/FacebookClientTest.php
index b9903184c..cf5153c17 100644
--- a/tests/FacebookClientTest.php
+++ b/tests/FacebookClientTest.php
@@ -24,7 +24,6 @@
 namespace Facebook\Tests;
 
 use Facebook\Exceptions\FacebookSDKException;
-use Facebook\Facebook;
 use Facebook\FacebookApp;
 use Facebook\FacebookRequest;
 use Facebook\FacebookBatchRequest;
@@ -165,7 +164,7 @@ public function testAFacebookBatchRequestWillProperlyBatchFiles()
 
         list($url, $method, $headers, $body) = $this->fbClient->prepareRequestMessage($fbBatchRequest);
 
-        $this->assertEquals(FacebookClient::BASE_GRAPH_VIDEO_URL . '/' . Facebook::DEFAULT_GRAPH_VERSION, $url);
+        $this->assertEquals(FacebookClient::BASE_GRAPH_VIDEO_URL, $url);
         $this->assertEquals('POST', $method);
         $this->assertContains('multipart/form-data; boundary=', $headers['Content-Type']);
         $this->assertContains('Content-Disposition: form-data; name="batch"', $body);
diff --git a/tests/FacebookRequestTest.php b/tests/FacebookRequestTest.php
index 6759e4da6..414b0f0f0 100755
--- a/tests/FacebookRequestTest.php
+++ b/tests/FacebookRequestTest.php
@@ -23,7 +23,6 @@
  */
 namespace Facebook\Tests;
 
-use Facebook\Facebook;
 use Facebook\FacebookApp;
 use Facebook\FacebookRequest;
 use Facebook\FileUpload\FacebookFile;
@@ -125,14 +124,14 @@ public function testAProperUrlWillBeGenerated()
 
         $getUrl = $getRequest->getUrl();
         $expectedParams = 'foo=bar&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9';
-        $expectedUrl = '/' . Facebook::DEFAULT_GRAPH_VERSION . '/foo?' . $expectedParams;
+        $expectedUrl = '/foo?' . $expectedParams;
 
         $this->assertEquals($expectedUrl, $getUrl);
 
-        $postRequest = new FacebookRequest($app, 'foo_token', 'POST', '/bar', ['foo' => 'bar']);
+        $postRequest = new FacebookRequest($app, 'foo_token', 'POST', '/bar', ['foo' => 'bar'], null, 'v0.0');
 
         $postUrl = $postRequest->getUrl();
-        $expectedUrl = '/' . Facebook::DEFAULT_GRAPH_VERSION . '/bar';
+        $expectedUrl = '/v0.0/bar';
 
         $this->assertEquals($expectedUrl, $postUrl);
     }
@@ -156,7 +155,7 @@ public function testAuthenticationParamsAreStrippedAndReapplied()
         $url = $request->getUrl();
 
         $expectedParams = 'bar=baz&access_token=foo_token&appsecret_proof=df4256903ba4e23636cc142117aa632133d75c642bd2a68955be1443bd14deb9';
-        $expectedUrl = '/' . Facebook::DEFAULT_GRAPH_VERSION . '/foo?' . $expectedParams;
+        $expectedUrl = '/foo?' . $expectedParams;
         $this->assertEquals($expectedUrl, $url);
 
         $params = $request->getParams();
diff --git a/tests/FacebookTest.php b/tests/FacebookTest.php
index 1ee717e66..ddab17ff5 100644
--- a/tests/FacebookTest.php
+++ b/tests/FacebookTest.php
@@ -29,7 +29,6 @@
 use Facebook\Authentication\AccessToken;
 use Facebook\GraphNodes\GraphEdge;
 use Facebook\Tests\Fixtures\FakeGraphApiForResumableUpload;
-use Facebook\Tests\Fixtures\FooBarPseudoRandomStringGenerator;
 use Facebook\Tests\Fixtures\FooClientInterface;
 use Facebook\Tests\Fixtures\FooPersistentDataInterface;
 use Facebook\Tests\Fixtures\FooUrlDetectionInterface;
@@ -39,6 +38,7 @@ class FacebookTest extends \PHPUnit_Framework_TestCase
     protected $config = [
         'app_id' => '1337',
         'app_secret' => 'foo_secret',
+        'default_graph_version' => 'v0.0',
     ];
 
     /**
@@ -50,6 +50,7 @@ public function testInstantiatingWithoutAppIdThrows()
         putenv(Facebook::APP_ID_ENV_NAME.'=');
         $config = [
             'app_secret' => 'foo_secret',
+            'default_graph_version' => 'v0.0',
         ];
         new Facebook($config);
     }
@@ -63,6 +64,19 @@ public function testInstantiatingWithoutAppSecretThrows()
         putenv(Facebook::APP_SECRET_ENV_NAME.'=');
         $config = [
             'app_id' => 'foo_id',
+            'default_graph_version' => 'v0.0',
+        ];
+        new Facebook($config);
+    }
+
+    /**
+     * @expectedException \InvalidArgumentException
+     */
+    public function testInstantiatingWithoutDefaultGraphVersionThrows()
+    {
+        $config = [
+            'app_id' => 'foo_id',
+            'app_secret' => 'foo_secret',
         ];
         new Facebook($config);
     }
diff --git a/tests/Helpers/FacebookCanvasHelperTest.php b/tests/Helpers/FacebookCanvasHelperTest.php
index a7506dd57..2396aa915 100644
--- a/tests/Helpers/FacebookCanvasHelperTest.php
+++ b/tests/Helpers/FacebookCanvasHelperTest.php
@@ -25,6 +25,7 @@
 
 use Facebook\FacebookApp;
 use Facebook\FacebookClient;
+use Facebook\Tests\FacebookTest;
 use Facebook\Helpers\FacebookCanvasHelper;
 
 class FacebookCanvasHelperTest extends \PHPUnit_Framework_TestCase
@@ -39,7 +40,7 @@ class FacebookCanvasHelperTest extends \PHPUnit_Framework_TestCase
     protected function setUp()
     {
         $app = new FacebookApp('123', 'foo_app_secret');
-        $this->helper = new FacebookCanvasHelper($app, new FacebookClient());
+        $this->helper = new FacebookCanvasHelper($app, new FacebookClient(), 'v0.0');
     }
 
     public function testSignedRequestDataCanBeRetrievedFromPostData()
diff --git a/tests/Helpers/FacebookJavaScriptHelperTest.php b/tests/Helpers/FacebookJavaScriptHelperTest.php
index abef58ec1..28d594e7e 100644
--- a/tests/Helpers/FacebookJavaScriptHelperTest.php
+++ b/tests/Helpers/FacebookJavaScriptHelperTest.php
@@ -25,6 +25,7 @@
 
 use Facebook\FacebookApp;
 use Facebook\FacebookClient;
+use Facebook\Tests\FacebookTest;
 use Facebook\Helpers\FacebookJavaScriptHelper;
 
 class FacebookJavaScriptHelperTest extends \PHPUnit_Framework_TestCase
@@ -36,7 +37,7 @@ public function testARawSignedRequestCanBeRetrievedFromCookieData()
         $_COOKIE['fbsr_123'] = $this->rawSignedRequestAuthorized;
 
         $app = new FacebookApp('123', 'foo_app_secret');
-        $helper = new FacebookJavaScriptHelper($app, new FacebookClient());
+        $helper = new FacebookJavaScriptHelper($app, new FacebookClient(), 'v0.0');
 
         $rawSignedRequest = $helper->getRawSignedRequest();
 
diff --git a/tests/Helpers/FacebookPageTabHelperTest.php b/tests/Helpers/FacebookPageTabHelperTest.php
index 29770342b..8ecd93bc1 100644
--- a/tests/Helpers/FacebookPageTabHelperTest.php
+++ b/tests/Helpers/FacebookPageTabHelperTest.php
@@ -25,6 +25,7 @@
 
 use Facebook\FacebookApp;
 use Facebook\FacebookClient;
+use Facebook\Tests\FacebookTest;
 use Facebook\Helpers\FacebookPageTabHelper;
 
 class FacebookPageTabHelperTest extends \PHPUnit_Framework_TestCase
@@ -36,7 +37,7 @@ public function testPageDataCanBeAccessed()
         $_POST['signed_request'] = $this->rawSignedRequestAuthorized;
 
         $app = new FacebookApp('123', 'foo_app_secret');
-        $helper = new FacebookPageTabHelper($app, new FacebookClient());
+        $helper = new FacebookPageTabHelper($app, new FacebookClient(), 'v0.0');
 
         $this->assertFalse($helper->isAdmin());
         $this->assertEquals('42', $helper->getPageId());
diff --git a/tests/Helpers/FacebookSignedRequestFromInputHelperTest.php b/tests/Helpers/FacebookSignedRequestFromInputHelperTest.php
index bfb3e0cf7..79cf182a8 100644
--- a/tests/Helpers/FacebookSignedRequestFromInputHelperTest.php
+++ b/tests/Helpers/FacebookSignedRequestFromInputHelperTest.php
@@ -24,6 +24,7 @@
 namespace Facebook\Tests\Helpers;
 
 use Facebook\FacebookApp;
+use Facebook\Tests\FacebookTest;
 use Facebook\Tests\Fixtures\FooSignedRequestHelper;
 use Facebook\Tests\Fixtures\FooSignedRequestHelperFacebookClient;
 
@@ -41,7 +42,7 @@ class FacebookSignedRequestFromInputHelperTest extends \PHPUnit_Framework_TestCa
     protected function setUp()
     {
         $app = new FacebookApp('123', 'foo_app_secret');
-        $this->helper = new FooSignedRequestHelper($app, new FooSignedRequestHelperFacebookClient());
+        $this->helper = new FooSignedRequestHelper($app, new FooSignedRequestHelperFacebookClient(), 'v0.0');
     }
 
     public function testSignedRequestDataCanBeRetrievedFromPostData()