Simulink Test harness inputs have wrong sample time

I'm trying to create a test harness for a Simulink model using Simulink Test. This model is discrete and sample time independent, so doesn't set any sample times. The config parameter sample time is set to auto. When I create a harness, the signal spec blocks and inports are set to a sample time of 5. There doesn't seem to be any way to specify this. Any ideas?

[JamieFish]

I've "solved" this problem by setting the model step size explicitly before generating the harness. Not sure why this is necessary.