diff --git a/src/Rest/GuzzleRestClient.php b/src/Rest/GuzzleRestClient.php
index 4a6c016..44c642d 100644
--- a/src/Rest/GuzzleRestClient.php
+++ b/src/Rest/GuzzleRestClient.php
@@ -114,4 +114,9 @@ private function containsTrailingSlash(string $baseUri): bool
     {
         return substr($baseUri, -1) === '/';
     }
+
+    public function refreshToken(): ?OAuthAccessToken
+    {
+        return null;
+    }
 }
diff --git a/src/Restforce.php b/src/Restforce.php
index 7683ba3..69517a3 100644
--- a/src/Restforce.php
+++ b/src/Restforce.php
@@ -119,7 +119,7 @@ public function query(string $queryString): ResponseInterface
         ]);
     }
 
-    public function refreshToken(): OAuthAccessToken
+    public function refreshToken(): ?OAuthAccessToken
     {
         return $this->getOAuthRestClient()->refreshToken();
     }
diff --git a/src/RestforceInterface.php b/src/RestforceInterface.php
index 4c1019a..a761e17 100644
--- a/src/RestforceInterface.php
+++ b/src/RestforceInterface.php
@@ -14,5 +14,5 @@ public function update(string $sobjectType, string $sobjectId, array $data): Res
     public function describe(string $sobjectType): ResponseInterface;
     public function find(string $sobjectType, string $sobjectId, array $fields = []): ResponseInterface;
     public function getNext(string $url): ResponseInterface;
-    public function refreshToken(): OAuthAccessToken;
+    public function refreshToken(): ?OAuthAccessToken;
 }