diff --git a/crates/ra_hir/src/ty.rs b/crates/ra_hir/src/ty.rs
index 3170ed24eb..9b71824859 100644
--- a/crates/ra_hir/src/ty.rs
+++ b/crates/ra_hir/src/ty.rs
@@ -453,6 +453,7 @@ impl Ty {
         {
             substs.push(Ty::Unknown);
         }
+        assert_eq!(substs.len(), def_generics.params.len());
         Substs(substs.into())
     }