an AC problem, about "The DAE is structurally inconsistent"

Hello everyone


I want to apply a sine voltage source to a piezoelectric ceramic. it's a model of COMSOL named"thickness_shear_quartz_oscillator.mph". the model works well but when I change the AC source to sine source, set the frequency 50Hz, same as AC. error occurs:

The DAE is structurally inconsistent

Variabels 2
Equation 0

you can see the model in the model library, hope you can help me

Thanks in Advance