Why does the input signal to a referenced model go to 0 when adding a level 2 s-function block to a referenced model?

I used this answer to create a dynamically limited integrator: Simulink Limit Integral Saturation Limit Dynamic Input . Added it to my model A which is referenced by model B. A single signal is used in B to feed A. If I attach a scope to it in B, all works fine. Otherwise the signal stays at 0 for all time. The only thing I changed was in model A I replaced a standard integrator block with my dynamic limited integrator. Why is the containing model signal suddenly not being calculated unless I put a scope on it? Mike