Formula Not Being Recognized as Such

I am having an occasional problem with an Excel formula not being recognized as a formula. This has happened with the Average and the Countif functions in two different files on two different computers. The current formula not being recognized is: =AVERAGE(CU:CU)

I have cleared the field by doing a Backspace and also by using Clear Contents. The first time I encountered this I actually deleted the row, added a new row and tried the formula again with the same results. I sent the file to a friend and she was successful. When I copied her version into my file the formula was not recognized.

Any ideas and/or suggestions would be greatly appreciated.

Thanks in advance.

BlindAlley,

Your suggestion worked like a charm. Thanks very much!