diff --git a/infer/src/backend/sil.ml b/infer/src/backend/sil.ml
index 6b46dfa31..99b0be59c 100644
--- a/infer/src/backend/sil.ml
+++ b/infer/src/backend/sil.ml
@@ -3831,6 +3831,7 @@ let rec expand_type tenv typ =
       begin
         match tenv_lookup tenv tname with
         | None -> assert false
+        | Some (Tvar _) -> assert false
         | Some typ' -> expand_type tenv typ'
       end
   | _ -> typ