VLookup nested in If/then statement

I'm trying to nest a vlookup in a if/then statement. I want to say the following:

if the vlookup does not find the result then give me "I" otherwise give me "C"

It looks as follows: '=IF(VLOOKUP(AA15,Sheet1!B:B,1,FALSE)="N/A","I","C")

I get the result of "C" when it finds AA15 in the column B on Sheet1, but I get N/A otherwise, I want "I".

Any ideas on what I'm doing wrong, please help. Thanks.

Thanks, I thought of that after I posted, awesome work, Thanks!