Background job aborts at MESSAGE W... - why?

Hi all,

there's a report I run in background task that aborts when processing the following line (during event START-OF-SELECTION):

MESSAGE w000 WITH sy-subrc lfa1-lifnr.

The job protocol shows up like this:

15.03.2007 10:33:23 Job wurde gestartet 00 516 S

15.03.2007 10:33:23 Step 001 gestartet (Programm Z 00 550 S

15.03.2007 10:40:07 Fehler Nr. 3 beim Senden an Li ZMAHN 000 E

15.03.2007 10:40:07 Job wurde nach System-Exceptio 00 564 A

As you see, the system interprets my warning message as an error message. But why?

Jörg