diff --git a/src/comp/middle/ty.rs b/src/comp/middle/ty.rs
index c8050eb1bc9e..deaf925f0b87 100644
--- a/src/comp/middle/ty.rs
+++ b/src/comp/middle/ty.rs
@@ -1943,7 +1943,12 @@ mod unify {
                 alt (struct(cx.tcx, expected)) {
                     case (ty::ty_var(?expected_id)) {
                         auto expected_n = expected_id as uint;
-                        union(cx, expected_n, actual_n);
+                        alt(union(cx, expected_n, actual_n)) {
+                            case (unres_ok) { /* fall through */ }
+                            case (unres_err(?t_e)) {
+                                ret ures_err(t_e);
+                            }
+                        }
                     }
                     case (_) {
 
diff --git a/src/test/compile-fail/binop-typeck.rs b/src/test/compile-fail/binop-typeck.rs
new file mode 100644
index 000000000000..5d48300e7427
--- /dev/null
+++ b/src/test/compile-fail/binop-typeck.rs
@@ -0,0 +1,9 @@
+// xfail-stage0
+// error-pattern:mismatched types
+// issue #500
+
+fn main() {
+  auto x = true;
+  auto y = 1;
+  auto z = x + y;
+}