Add assert to prevent regression

test/Ownable.js

@@ -38,6 +38,7 @@ contract('Ownable', function(accounts) {
     let originalOwner = await ownable.owner();
     try {
       await ownable.transferOwnership(null, {from: originalOwner});
+      assert.fail()
     } catch(error) {
       assertJump(error);
     }