Error: unknown element type 0 in nonlinear analysis

[whelanmj]

I have built a model under linear natural frequency analysis but now have to convert it to a nonlinear natural frequency analysis because I need to use shell elements to provide out-of-plane bending.  However, after I change the analysis type and re-specify the element types and element properties, I get an error during the "Start to make nodal condition tables . . ." that "Error: unknwon element type 0 in nonlinear analysis".

Does anyone know what might be causing this?  What is element type 0?  I have tried disabling elements to try to track down the "culprit" but none of them seem to be the cause (I'm just using shells, beams, and springs).  I really don't want to have to redraw my model because it is quite large and took a long time to initially develop.  

[S.LI]

I guess the error means that some of elements have no appropriate element types. You need to double check your part setting and "check model" again.