Buildings Boiler Polynomial ask for modelica.interfaces.partialmedium

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Dear all, I'm new to modelica and especially to the buildings modelica library. I'm using the Buildings library in OpenModelica under Ubuntu. I'm trying to build a simple model with a boiler but when I use the Buildings.Fluid.Boilers.BoilerPolynomial the following error shows up component boi contains the definition of a partial class Medium. Please redeclare it to any package compatible with Modelica.Media.Interfaces.PartialMedium.

this is the definition in the model package Medium = Buildings.Media.Water; Buildings.Fluid.Boilers.BoilerPolynomial boi(redeclare package Medium = Medium)

what I'm missing?