check valve thermal liquid negative area

I'm trying to model a circuit using thermal liquid blocks. Within the circuit I have a heat exchanger and in parallel with it there are 2 check valves with a cracking pressure differential of 2 bar and a maximum opening pressure differential of 2.5 bar. The check valves are there to prevent the pressure drop over the heat exchanger to become too large. The flow through the heat exchanger changes direction regularly (hence 2 check valves, one for each direction). When running the model it regularly crashes with the following error message:

Argument of sqrt must be nonnegative. The assertion comes from:

Block path: RDN/CV2

Assert location:

o (location information is protected)

Playing around with the smoothing factor and adding small volumes on either side of the check valves sometimes helps, but in general it just moves the time at which the model crashes a little. When the model crashes the results in the explorer show a negative area for the valve.

Does anyone know how to make the modelling of check valves in thermal liquids more stable (or how to avoid negative valve areas)?

Thanks

Joe