diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index f4747b5b108a..941cef39fb25 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -1614,7 +1614,7 @@ fn assemble_candidates_for_unsizing(&mut self,
                 //
                 // We always upcast when we can because of reason
                 // #2 (region bounds).
-                data_a.principal.def_id() == data_a.principal.def_id() &&
+                data_a.principal.def_id() == data_b.principal.def_id() &&
                 data_a.builtin_bounds.is_superset(&data_b.builtin_bounds)
             }