I've looked and read, but haven't found solution . . .

I'm using a combo box in a worksheet, not a user form. I've followed the
very helpful CONTEXTURES instructions, but when the combo box is selected at
double click in the cell, the combo box opens blank, and any value can be
entered without data validation error (what's typed is not in the list range
of valid values).

Validation for the cell alone works (if the combo box isn't used). The
combo box contains all the valid values and type-ahead works -- it's just
that when it opens, the blank space allows any entry and data validation
doesn't pick up on it if entered using the combo box.

Can I default to the first value (in a worksheet, not a form) and/or can I
make the free-form user entry error out? This falls under category of
"simple", "not easy" -- at least for a knuckle-draggin' non-technical
supervisor, . . . like me. Please advise.