Runtime Error 3320 (1 Viewer)

"Function is not available in expressions in table level validations"


I keep getting this message when trying to update the value of a text box when using the following code

Forms!frmFinanceProposal!Text940 = Me.Text253

Would someone be able to explain what this message means pls!!!

Have a look at this :
http://www.access-programmers.co.uk/forums/showthread.php?t=85277