S Function with 0 output ports - callbacks mdlOutputs and mdlGetTimeOfNextVarHit never called

My S function C program works fine when there are S function blocks with nonzero output ports. But for 0 output ports if I set ssSetNumOutputPorts(S,0), the callbacks such as mdlOutputs and mdlGetTimeOfNextVarHit are never get called. Is this the right behavior? Or am I doing something wrong?

Thanks in advance.