How to specify a NULL Bus Object to an S-Function input?

I have created an S-Function that takes a pointer to a bus object for one of its parameters. The parameter is optional. If the pointer address is NULL, then the function changes it's behavior.

I want to create a NULL bus object argument in Simulink to drive the input. I have tried many different things, but I either get build errors or Simulink makes a structure anyway with an address that is not NULL.

How do I specify a NULL input to an S-Function?