The number in this cell is formatted as text or preceded by an apostrophe
 

I'm creating an excel 10 worksheet in vb .net. If I format a range with

oSheet.Range("G1", "Y1").NumberFormat = "0.00"

the finished worksheet shows the little green triangle in each cell in the
range, and it gives me the message "The number in this cell is formatted as
text or preceded by an apostrophe".

How do I turn these cells back into numbers with the proper numeric format?

Jeremy