OpenModelica: Using pumps

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I'm trying to create a model that uses a pump to pump a fluid from a low pressure environment to a high pressure environment. The source and sink are both Boundary_pT and the pump is connected to two static pipes which are then connected to these boundaries. I get the following error:

"Wrong pump curve -- head_nominal must be monotonically decreasing with increasing V_flow_nominal"

How do I fix this?

Thanks!