Excel 2007 : Problems with ISNA formula

Hi everyone,

This is probably a simple thing, but can someone tell me what is wrong with this formula?

=if(ISNA(VLOOKUP(B2,[SCHEDULE.xlsx]August!$O$81,10,FALSE),"",VLOOKUP(B2,[SCHEDULE.xlsx]August!$O$81,10,FALSE)))

It keeps highlighting the ,"", portion, but I can't seem to get to the bottom of the issue.

Thanks!

Karen