Hello!
I did some search, looks like this error message is generated when a code check during a key length calculation in NAT code. If the value returned is "0" then the error message is generated.
For example, and from what I saw this can often happen when we process a wrong packet with ip 0.0.0.0, that could be caused by a buggy packet sent to the router, or because of a bug on the router.
You can double check this by doing a "debug ip nat detail" and see what NAT was doing just before the error message, however this is a very chatty debug and you need to be cautious if you are going to try that in production.
If the error message occurs very often, I guess it's worth to already try to put a deny 0.0.0.0 in the acl attached to NAT and see if that stops. If not, then you probably need to go for controlled debugging session to find the guilty packets.
Thanks,
Raphael