DLookup On A Field

In a previous database i used the dlookup function as the conrol source for an unbound text box.
it looked like this:

=DLookUp("[Name]","tbl_GP_Details","[GPNumber] =" & [Forms]![frmPatient_Dets]![GPNumber])

This populated the text box with the name from the gp details where the gp number was the same as the gp number on the form.

This worked fine but now i need to create something similar however instead of using a number to link them (GPNumber) the field is text this time, the above code does not work. Do i have to add anything else so it recognizes it to be a text field and not numeric??

Hope this makes sense