diff --git a/test/common/resolver_test.py b/test/common/resolver_test.py
index 5c5bc03..57fc626 100644
--- a/test/common/resolver_test.py
+++ b/test/common/resolver_test.py
@@ -6,6 +6,16 @@
 
 
 class TestStringResolver(unittest.TestCase):
+    def test_resolve_no_custom_vars(self):
+        # given:
+        init_str_resolver("1.0", "token123")
+        init_str_resolver_custom_variables(None)
+
+        # when:
+        resolved_string = resolve("Testing {VERSION}")
+
+        # then:
+        self.assertEqual("Testing 1.0", resolved_string)
 
     def test_resolve_predefined(self):
         # given:
diff --git a/valhalla/common/resolver.py b/valhalla/common/resolver.py
index bdaf7c0..d213d59 100644
--- a/valhalla/common/resolver.py
+++ b/valhalla/common/resolver.py
@@ -29,7 +29,8 @@ def init_str_resolver(version: str, token: str):
 
 def init_str_resolver_custom_variables(variables: dict):
     global CUSTOM_VARIABLES_DICT
-    CUSTOM_VARIABLES_DICT.update(variables)
+
+    CUSTOM_VARIABLES_DICT.update({} if variables is None else variables)
 
     for key, value in CUSTOM_VARIABLES_DICT.items():
         info(f"Custom variable: {key} set to: {value}")