my_hdr as unknown command rejected

[Anca Tibor- Attila <tibor-attila.anca@xxxxxxxxxxxxx>]

Hi,

I tried to include my_hdr in muttrc, but mutt always said, it does not 
know this command:

my_hdr X-Operating-System: SuSE 8.2 Professional.

I took this from a sample-muttrc from the net. What's wrong with it?

Thanks,
Tibor