Invalid references in a formula message when creating a chart
 

I am trying to create a chart showing the percentages of title exams
completed by month. When I select the data and try to create a chart I
receive a error message indicating that "a formula in the worksheet contains
invalid references". My individual cells do not reflect any error flags and
I have run the error checking tool on each of the worksheets in the workbook
and there are no errors.

What could be causing this?