Modules & VBA :: External Name Not Defined

I use the following code behind the BeforeUpdate event of a control, to validate input.

Code:
Private Sub StockPrice_BeforeUpdate(Cancel As Integer)
'strings used for the MsgBox
Dim strTitle As String
Dim strMsg1 As String
Dim strMsg2 As String
Dim strMsg3 As String

[Code] ....

There are two different forms that use that control and this code behind the BeforeUpdate event.

It works on one but not on the other, allowing to input anything in the control with checking and without bringing up any msgbox.
Searching what's the problem in the immediate window, brings up a message saying "External Name not defined'.