diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index aa4ab9c7ee9..5e9a28b5cce 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1732,6 +1732,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         env_predicate: PolyProjectionPredicate<'tcx>,
         potentially_unnormalized_candidates: bool,
     ) -> ProjectionMatchesProjection {
+        debug_assert_eq!(obligation.predicate.def_id, env_predicate.projection_def_id());
+
         let mut nested_obligations = Vec::new();
         let infer_predicate = self.infcx.instantiate_binder_with_fresh_vars(
             obligation.cause.span,