diff --git a/src/DefinitionResolver.php b/src/DefinitionResolver.php
index 5a6f88b..2ed66f6 100644
--- a/src/DefinitionResolver.php
+++ b/src/DefinitionResolver.php
@@ -833,7 +833,9 @@ class DefinitionResolver
             // Unknown
             return new Types\Mixed;
         }
-        return null;
+        // Return Types\Mixed in case nothing was found
+        // TODO: Find cases that don't match any rule and handle them'
+        return new Types\Mixed;
     }
 
     /**