problem using OPTION ARB_fog_linear in fragment program

I have been writing some simple normalmap bump effect with ARB_fragment_program, and I tried to add fog effect to it. I am using
!!ARBfp1.0 OPTION ARB_fog_linear;
in my fragment program, but I can’t see any fog effect on polygons using it. Other polygons using fixed pipeline are correctly fogged. I can’t figure out why. Is there anything more needs to be done?

not sure, but likely you need to output fogcoord in your vertex program

Tou definitely need to do that.