Error Message within Monitor

We are receiving the following error message:

Character representation of a number value is in error (C G D F).  This
error occurs at statement 4397 of the code listed below.  We know what the
error is, but we do no understand why the Monitor is not catching it.


The program code is:

4396 MONITOR;
4397 NUMBER=%DEC(@IDSC : 3 : 0);
4398 ON-ERROR *ALL;
4399 LEAVESR;
4400 ENDMON;

There are times that the value is @IDSC is not numeric.  Most of the time
the error appears to be caught and the subroutine is exited.
Several times in the past few months we get the message shown above that
monitor has not dealt with.

Can you offer an explanation as to why this periodically occurs?

Thanks

Norris