[Mimedefang] Error In filter

[Evil - Chilean Arrogance && Insanity]

Hi!
I have some errors with one filter the "filter_sender" in the log appears 
this message.

mimedefang-multiplexor: Slave 0 stderr: Use of uninitialized value in 
integer gt (>) at /etc/mail/mimedefang-filter line 299, <STDIN> line 1. 

and here is the code, somebody know a solution?? i can't find it..
please help!!
if ($sender =~ /prueba\@destroyer/i) {
                if ( -s './INPUTMSG' > 1000*1024) {
                        return ('REJECT','Message Big');

}
}