Hi there,
I had the same error and whilst I appreciate this is 7 years too late I thought it'd be best to post this reply in case anyone else encountered a similar error.
I found that I had accidentally added some controls to my character set that were being driven by other parts of the rig I was working on and obviously you can't set a key on something that is being driven by another control. What happened then is that anything in the character set order that was below the problem control was also not being set as a result and throwing up the error you have received.
So I removed my character set and ensured that i only had the controls and attributes of those controls that i was going to use before creating a new character set and now the rig works just fine and i don't have to manually key everything in before hand. So if you have any character controls that have constraints, set driven keys, are being driven through the relationship editor or have expressions on them I recommend you remove those attributes before you create your character set or you can remove them in the character set editor by going to Window > Relationship Editors > Character Sets (Maya 2016).
As I said i know i'm a bit late to the party but i thought it was worth posting here as this is the first post that comes up when you google this issue.
Thank you and good day!