I have a fairly simple issue but am not quite sure what the solution is:

Have created a basic form in Excel with several combo boxes.
The combo drop down values are "Y" or "N".

I have a command button that adds the values to a worksheet and clears the
combo boxes.

How do I prevent users from adding blank values to the worksheet? Ideally,
I would like to create an error routine/procedure which informs the user that
no blank values are allowed and highlights (or moves the cursor to) the field
with the blank value.

Thanks.

JPA.