[fix] fix this.callee assumption

lib/dialects/mysql/query.js

@@ -115,7 +115,7 @@ module.exports = (function() {
         var values = match[1].split('-')
           , fields = {}
           , message = 'Validation error'
-          , uniqueKey = this.callee.__options.uniqueKeys[match[2]];
+          , uniqueKey = this.callee && this.callee.__options.uniqueKeys[match[2]];
 
         if (!!uniqueKey) {
           if (!!uniqueKey.msg) message = uniqueKey.msg;