diff --git a/src/boot/me/typestate.ml b/src/boot/me/typestate.ml
index 764fdc96154..2c0c4b156af 100644
--- a/src/boot/me/typestate.ml
+++ b/src/boot/me/typestate.ml
@@ -68,9 +68,10 @@ let determine_constr_key
                           if referent_is_slot cx aid
                           then
                             if type_has_state
-                              (slot_ty (get_slot cx aid))
+                              (strip_mutable_or_constrained_ty
+                                 (slot_ty (get_slot cx aid)))
                             then err (Some aid)
-                              "predicate applied to slot of mutable type"
+                              "predicate applied to slot of state type"
                             else aid
                           else
                             (* Items are always constant, they're ok.