Check for an empty validation error message. Closes #2009

lib/instance-validator.js

@@ -329,7 +329,7 @@ InstanceValidator.prototype._invokeBuiltinValidator = Promise.method(function(va
   // extract extra arguments for the validator
   // extract extra arguments for the validator
   var validatorArgs = test.hasOwnProperty('args') ? test.args : test;
   var validatorArgs = test.hasOwnProperty('args') ? test.args : test;
   // extract the error msg
   // extract the error msg
-  var errorMessage = test.hasOwnProperty('msg') ? test.msg :
+  var errorMessage = test.hasOwnProperty('msg') && test.msg ? test.msg :
     'Validation ' + validatorType + ' failed';
     'Validation ' + validatorType + ' failed';