Anomaly: Uncaught exception Not_found(_). Please report. (polyproj)

[JasonGross]

Inductive paths A (x : A) : A -> Type := idpath : paths A x x.
Notation "x = y" := (@paths _ x y) : type_scope.

Axioms A B : Type.
Axiom P : A = B.
Definition foo : (A = B) * (A = B).
split; abstract (rewrite <- P; reflexivity).
Defined. (* Anomaly: Uncaught exception Not_found(_). Please report. *)

There's another error that I get on the rewrite, sometimes, but I'm still working on a test case for that one.