Data validation dropdown: works in one sheet, not in another
 

I defined a pulldown validation list in Sheet3.
I inserted the reference to the list in Sheet1 and it works fine.
I inserted the reference to the list in Sheet2 and I'm told "the source
currently evaluates to an error".
The formatting of the relevant cells in Sheet1 and Sheet2 is identical.
Sheet and workbook protection is off in all sheets. Yes, I used the "="
(=ThruList).
Can anyone tell me why it works in one sheet and not the other?
---Pat Riley