I can call my component as com but not as com+

hi,
I have a com (dll) component.if I register it through regsvr32 then I can
add reference to my c# project and use it without no problem.(vs.2003,win
xp)

bu if I install it into mts and add reference again I get "a null pointer
passed to the stub" error.I have the problem in function call
line.powerbuilder 9 can call the component from mts(com+) without problem.

COClass_uo_service_homeClass mycom= new COClass_uo_service_homeClass();

Array arr1=new Array[1500];

arr1=mycom.uof_get_root_services(1); /////// this line I get error



What am I missing?.thanks for help.