Seems like this should be a VERY SIMPLE thing to do, but I'm at my wit's end!

I have a responsive form with just one field - the Title field. The SharePoint list column is restricted to 255 characters, but the Nintex form will allow the user to input more than 255 characters and submit it - it just truncates the entry when it writes to the list with no notice to the user.

I've tried using a Rule on the field, such as When length({Self})>255, Then Invalidate with appropriate Validtion message, but this doesn't appear to do anything; it accepts an entry longer than 255 characters, but truncates it in the SharePoint list. 

I also tried Validation in the Control Settings, using a Regular Expression:

^

But this gives the error message no matter how long or short the entry is.

Appreciate any help!