Fitter ignoring PLL compensation assignment as invalid

I'm using 10.1, and when I say ALTLVDS, I mean either ALTLVDS_TX or ALTLVDS_RX. I created a test design with both, and shared the PLL between them, and verified in the Fitter report for the PLL Summary that the mode is set to Normal mode, not Source Synchronous. It makes no sense that it would set it to Source Synchronous, and then tell you that the assignment is invalid. I have no idea why you are seeing that. I guess at this point, I would suggest you delete that instance and try recreating a new one with the MegaWizard. If you are only using the ALTLVDS_RX, or both but not sharing the PLL, then I didn't test that and it could be a bug, but I'd be surprised.