Subject of the issue

WYSIWYG plugins causes "TypeError: string is undefined"

Your environment

• version of jquery-validate: 1.18.0
• which browser and its version: Chrome 70.0.3538.67 x64 and Firefox 62.0.3 x64

Steps to reproduce

Using WYSIWYG (tested with Gijgo and TinyMCE) type anything on text field.
Click outside.

Expected behaviour

Just no Errors

Actual behaviour

Logging Error: TypeError: string is undefined
Line 1023

Fix suggestion

Just force variable string to be string everytime on line 1023:
Replace:

1023 return string.replace..
To:
1023 return (string+"").replace..