Errors when using datepicker

[juser]

I am using CKForms 1.3.3 with Joomla 1.5
I have a form that I am implementing using the plugin so that I can display the form in an article.

All well until I activated the datepicker, I am now getting the following error:

"Notice: Undefined index: lang in C:\wamp\www\cormerant\plugins\content\ckforms.php on line 635"

The form itself still displays everything OK including the datepicker but the error is in full view of visitors to the site so it is a bit of a showstopper.

The error dissappears when the datepicker is de-activated.

Appreciate some help or ideas with this

Thanks