[Chimera-users] when will NoResidueRotamersError be raised?

I would like to mutate one residue to another. So far, the only way I found is to use the rotamers of the other residues on the original. I'm using chimera's python script. I would like to know if the NoResidueRotamersError will only be raised for residues that don't have rotamers like GLY. If not, then how can I replace the amino acid residue that I want to mutate? I am using the "getRotamers" function, which returns this error. XBL