diff --git a/src/ir.rs b/src/ir.rs
index 71cd254..99f0b45 100644
--- a/src/ir.rs
+++ b/src/ir.rs
@@ -688,7 +688,7 @@ fn downgrade_inherit(
                 .ir())
             },
         )?;
-        stcs.insert(ident, expr).unwrap();
+        assert!(stcs.insert(ident, expr).is_none());
     }
     Ok(())
 }