Fatal error: exception Not_found

[yallop]

discovered by @pxeger:

$ OCAMLRUNPARAM=b ocaml 
        OCaml version 4.02.1+dev0-2014-08-29

# module type ModuleType = sig module type T end;;
module type ModuleType = sig module type T end
# let test {M1: ModuleType} (f: {M1: M1.T} -> 'a -> 'b) = ();;
val test : {M1 : ModuleType} -> ({M1 : M1.T} -> 'a -> 'b) -> unit = <fun>
# module type X = sig end;;
module type X = sig  end
# let () = test (fun {X: X} x -> X);;
Fatal error: exception Not_found
Raised at file "typing/env.ml", line 382, characters 18-27
Called from file "typing/env.ml", line 659, characters 17-35
Called from file "typing/env.ml", line 1771, characters 20-45
Called from file "typing/typetexp.ml", line 916, characters 13-49
Called from file "parsing/location.ml", line 344, characters 2-13
Called from file "typing/typetexp.ml", line 1027, characters 13-67
Called from file "parsing/location.ml", line 314, characters 14-19
Called from file "parsing/location.ml", line 367, characters 8-24
Called from file "toplevel/toploop.ml", line 491, characters 11-42
Called from file "toplevel/topstart.ml", line 13, characters 8-22