anistropic material ERROR in HFSS

when a component was defined as mu=(1,-1,1),My HFSS model can run smoothly. However, if I redefine it as mu=(1,1,-1) or mu=(-1,1,1),there will be error, read:
[error] Discrete solution setup, process abc3d : Error in finite element matrix generation in subprocess "Zero diagonal entry in matrix.". Please check to make sure that a valid model has been created.