Validation Text ignored in Access 2013 web app

Hi,

I'm moving a desktop Access 2013 DB to a SharePoint Online Access web app and having trouble with the field Validation Text message. When an incorrect entry is made (List or Datasheet view) I get the following system message instead of the Validation Text message I entered.

"Access Services
X While converting a value from one data type to another, Access produced a value that can't be stored."

Correct values entered in the field don't receive this message when record is saved so the validation rule is working correctly.

Thanks,

Mark