We're not really sure.
There's some advice here: http://logical.saclay.inria.fr/coq-puma/messages/883f1468f299699c
Try using destruct.
Note that this error message is sensitive to what's in the context. Try using clear to remove any hypotheses you don't need.
Also try dependent induction from Coq.Program.Equality (although this requires additional axioms outside of CiC).