pressure with isentropic fails

Summary

Steps to reproduce

Following fails in e.g. rhoSimpleFoam tutorial:

isentropicPressure
{
    type            pressure;
    libs            ("libfieldFunctionObjects.so");
    enabled         yes;
    writeControl    writeTime;
    calcIsen        yes;
    calcTotal       no;
    calcCoeff       no;
    result          isenTropicP;
}

Example case

compressible/rhoSimpleFoam/squareBend