Can't update field on form

I gave a field on my form named GirlID which is set to Text in the table. When I try to enter data into it I get the error message; Can't update field. Then after 2 tries it let's me enter the data. Here is the only code tun on this field. There is a max number of 20 set in the table of field length but the data in the field never approaches that.

Private Sub GirlID_AfterUpdate()
If DCount("GirlID", "tbl_main", "[GirlID] = '" & Me.GirlID & "'") > 0 Then

MsgBox "This Girl Already Exists! Try Again?"
'GirlID = StrConv(GirlID, vbProperCase)
End If
End Sub