I am getting warning as "WARNING: Invalid cp (-5.727386e-02 J/kgK) for methane-liq at temperature 263.457829 K". I am using polynomial for defining cp.
Please cross-check your polynomial coefficients for specific heat. Please make sure that the polynomial relation, you are entering, is correct and it gives realistic value for specific heat. Sometimes, it may give the wrong value as the variable value is changed. If specific heat goes negative then solver clips it at zero and this clipped value is used.
Please refer to the attached document below for more information.