IFN and division by zero

I have the following code:

data PIRsum2A;
set PIRsum2;
pirrev=ifn(nights=0,0,revenue/nights);
run;

I thought that the ifn statement would prevent division by zero but it
generates a note in the log anyway.

NOTE: Division by zero detected at line 16 column 32.
CLUB_RESERVATIO_NO=85589101 CLUB_OWNERSHIP_ID=290009001 nights=0 revenue=0
pirrev=0 _ERROR_=1 _N_=4723

Then the error "Limit set by ERRORS= option reached. Further errors of this
type will not be printed."

The output is unaffected but the errors are not acceptable.

Have others had this problem?

Thanks.