diff --git a/lib/Unexpected.js b/lib/Unexpected.js
index 962749a07..71fa6809e 100644
--- a/lib/Unexpected.js
+++ b/lib/Unexpected.js
@@ -917,7 +917,7 @@ Unexpected.prototype.lookupAssertionRule = function (subject, testDescriptionStr
     }
     var handlers = this.assertions[testDescriptionString];
     if (!handlers) {
-        return;
+        return null;
     }
     var cachedTypes = {};
 
@@ -986,6 +986,8 @@ Unexpected.prototype.lookupAssertionRule = function (subject, testDescriptionStr
             return handler;
         }
     }
+
+    return null;
 };
 
 function makeExpectFunction(unexpected) {