Messagebox triggers before the Focus gets to object

I have a excel data entry form that has several objects; combo, text boxes, check boxes etc.. One of the objects is combo box, which has a sub routine that validates the input, so an option has to be chosen. The problem is that before the combo box gets the focus it pops the message up!? The tab order goes from a Frame containing two Option Buttons than another Frame with 2 disable option buttons that are disable, then when I tab out of the frame in to the combo box the message a window pops up with the alert of not entering a value. Does anybody have an idea what is triggering the msgbox to pop before the focus gets to the cbobox?
This is the code for the combo box;

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Also I changed the tab order to skip the disable Frame (with 2 option buttons) and the combo box works fine waits for the right input if it does tabs to the next object, but if no value is entered it pops the message box alert.
What is the disable frame doing to make combo box to trigger the message box?
Any input will be greatly appreciated.

I'd try putting the validation in the ComboBox_BeforeUpdate event.