diff --git a/util/joint_sort.hh b/util/joint_sort.hh
index 13a52b67b..b1ec48e26 100644
--- a/util/joint_sort.hh
+++ b/util/joint_sort.hh
@@ -40,6 +40,12 @@ template <class KeyIter, class ValueIter> class JointIter {
       swap(first.value_, second.value_);
     }
 
+    void DeepSwap(JointIter &other) {
+      using std::swap;
+      swap(*key_, *other.key_);
+      swap(*value_, *other.value_);
+    }
+
   private:
     friend class JointProxy<KeyIter, ValueIter>;
     KeyIter key_;
@@ -84,10 +90,7 @@ template <class KeyIter, class ValueIter> class JointProxy {
     }
 
     friend void swap(JointProxy<KeyIter, ValueIter> first, JointProxy<KeyIter, ValueIter> second) {
-      // Allow argument-dependent lookup.
-      using std::swap;
-      swap(*first.inner_.key_, *second.inner_.key_);
-      swap(*first.inner_.value_, *second.inner_.value_);
+      first.Inner().DeepSwap(second.Inner());
     }
 
   private:
diff --git a/util/joint_sort_test.cc b/util/joint_sort_test.cc
index 4dc859164..b24c602c9 100644
--- a/util/joint_sort_test.cc
+++ b/util/joint_sort_test.cc
@@ -47,4 +47,16 @@ BOOST_AUTO_TEST_CASE(char_int) {
   BOOST_CHECK_EQUAL(327, values[3]);
 }
 
+BOOST_AUTO_TEST_CASE(swap_proxy) {
+  char keys[2] = {0, 1};
+  int values[2] = {2, 3};
+  detail::JointProxy<char *, int *> first(keys, values);
+  detail::JointProxy<char *, int *> second(keys + 1, values + 1);
+  swap(first, second);
+  BOOST_CHECK_EQUAL(1, keys[0]);
+  BOOST_CHECK_EQUAL(0, keys[1]);
+  BOOST_CHECK_EQUAL(3, values[0]);
+  BOOST_CHECK_EQUAL(2, values[1]);
+}
+
 }} // namespace anonymous util