Error when generating S-function from subsystem with Stateflow chart and Function Call outputs

I am getting an error when generating a S-function for a subsystem containing a Stateflow chart with Event outputs to drive other function call subsystems. On my subsystem I right-click->C/C++ Code->Generate S-Function. The "Generate S-Function for Subsystem: XXX" dialog box opens and I select "Build". I get the following error:

   Invalid "function-call" connection.

   Function-call initiator 'XXX/ SFunction ' is  attempting to invoke a function-call
   block on output port 1, element 1 even though this port element has not been
   configured to output a function-call signal.

   Invalid connection ends with 'XXX/A' which is illegal because it is not 
   a function-call block.

Is there any way to generate an S-Function from a subsystem with function call outputs?