diff --git a/parallel_hashmap/phmap.h b/parallel_hashmap/phmap.h
index 3d11e0e..48173fe 100644
--- a/parallel_hashmap/phmap.h
+++ b/parallel_hashmap/phmap.h
@@ -3372,6 +3372,11 @@ public:
         fCallback(set);
     }
 
+    // unsafe, for internal use only
+    Inner& get_inner(size_t idx) {
+        return  sets_[idx];
+    }
+
     // Extension API: support for heterogeneous keys.
     //
     //   std::unordered_set<std::string> s;
@@ -3473,15 +3478,20 @@ public:
         return it == end() ? node_type() : extract(const_iterator{it});
     }
 
-    void swap(parallel_hash_set& that) noexcept(
-        IsNoThrowSwappable<EmbeddedSet>() &&
-        (!AllocTraits::propagate_on_container_swap::value ||
-         IsNoThrowSwappable<allocator_type>())) {
+    template<class Mtx2_>
+    void swap(parallel_hash_set<N, RefSet, Mtx2_, Policy, Hash, Eq, Alloc>& that)
+        noexcept(IsNoThrowSwappable<EmbeddedSet>() &&
+                 (!AllocTraits::propagate_on_container_swap::value ||
+                  IsNoThrowSwappable<allocator_type>()))
+    {
         using std::swap;
+        using Lockable2 = phmap::LockableImpl<Mtx2_>;
+         
         for (size_t i=0; i<num_tables; ++i)
         {
-            typename Lockable::UniqueLocks l(sets_[i], that.sets_[i]);
-            swap(sets_[i].set_, that.sets_[i].set_);
+            typename Lockable::UniqueLock l(sets_[i]);
+            typename Lockable2::UniqueLock l2(that.get_inner(i));
+            swap(sets_[i].set_, that.get_inner(i).set_);
         }
     }
 
@@ -3620,8 +3630,11 @@ public:
         return !(a == b);
     }
 
+    template<class Mtx2_>
     friend void swap(parallel_hash_set& a,
-                     parallel_hash_set& b) noexcept(noexcept(a.swap(b))) {
+                     parallel_hash_set<N, RefSet, Mtx2_, Policy, Hash, Eq, Alloc>& b)
+        noexcept(noexcept(a.swap(b)))
+    {
         a.swap(b);
     }
 
@@ -3700,14 +3713,16 @@ private:
 
     // TODO(alkis): Optimize this assuming *this and that don't overlap.
     // --------------------------------------------------------------------
-    parallel_hash_set& move_assign(parallel_hash_set&& that, std::true_type) {
-        parallel_hash_set tmp(std::move(that));
+    template<class Mtx2_>
+    parallel_hash_set& move_assign(parallel_hash_set<N, RefSet, Mtx2_, Policy, Hash, Eq, Alloc>&& that, std::true_type) {
+        parallel_hash_set<N, RefSet, Mtx2_, Policy, Hash, Eq, Alloc> tmp(std::move(that));
         swap(tmp);
         return *this;
     }
 
-    parallel_hash_set& move_assign(parallel_hash_set&& that, std::false_type) {
-        parallel_hash_set tmp(std::move(that), alloc_ref());
+    template<class Mtx2_>
+    parallel_hash_set& move_assign(parallel_hash_set<N, RefSet, Mtx2_, Policy, Hash, Eq, Alloc>&& that, std::false_type) {
+        parallel_hash_set<N, RefSet, Mtx2_, Policy, Hash, Eq, Alloc> tmp(std::move(that), alloc_ref());
         swap(tmp);
         return *this;
     }
diff --git a/tests/flat_hash_map_test.cc b/tests/flat_hash_map_test.cc
index d6d50d2..f64bbdd 100644
--- a/tests/flat_hash_map_test.cc
+++ b/tests/flat_hash_map_test.cc
@@ -22,6 +22,10 @@
     #define THIS_EXTRA_TPL_PARAMS 
 #endif
 
+#ifndef THIS_EXTRA_TPL_PARAMS_NULLMUTEX
+    #define THIS_EXTRA_TPL_PARAMS_NULLMUTEX 
+#endif
+
 #include "parallel_hashmap/phmap.h"
 
 #if defined(PHMAP_HAVE_STD_ANY)
@@ -62,6 +66,12 @@ template <class K, class V, class H = phmap::priv::hash_default_hash<K>,
           class Alloc =  phmap::priv::Allocator<
               phmap::priv::Pair<const K, V>>>
 using ThisMap = THIS_HASH_MAP<K, V, H, Eq, Alloc THIS_EXTRA_TPL_PARAMS>;
+    
+template <class K, class V, class H = phmap::priv::hash_default_hash<K>,
+          class Eq = phmap::priv::hash_default_eq<K>,
+          class Alloc =  phmap::priv::Allocator<
+              phmap::priv::Pair<const K, V>>>
+using ThisMap_NullMutex = THIS_HASH_MAP<K, V, H, Eq, Alloc THIS_EXTRA_TPL_PARAMS_NULLMUTEX>;
 
 static_assert(!std::is_standard_layout<NonStandardLayout>(), "");
 
diff --git a/tests/parallel_flat_hash_map_mutex_test.cc b/tests/parallel_flat_hash_map_mutex_test.cc
index 73dbf7b..e88e7da 100644
--- a/tests/parallel_flat_hash_map_mutex_test.cc
+++ b/tests/parallel_flat_hash_map_mutex_test.cc
@@ -9,4 +9,6 @@
     #define THIS_EXTRA_TPL_PARAMS , 4, boost::upgrade_mutex
 #endif
 
+#define THIS_EXTRA_TPL_PARAMS_NULLMUTEX , 4, phmap::NullMutex
+
 #include "parallel_hash_map_test.cc"
diff --git a/tests/parallel_hash_map_test.cc b/tests/parallel_hash_map_test.cc
index 4812e06..0188691 100644
--- a/tests/parallel_hash_map_test.cc
+++ b/tests/parallel_hash_map_test.cc
@@ -9,6 +9,24 @@ namespace phmap {
 namespace priv {
 namespace {
 
+TEST(THIS_TEST_NAME, Swap) {
+    using Map = ThisMap<int, int>;
+    using MapB = ThisMap_NullMutex<int, int>;
+    
+    Map t;
+    EXPECT_TRUE(t.find(0) == t.end());
+    auto res = t.emplace(0, 1);
+    EXPECT_TRUE(res.second);
+    EXPECT_EQ(1, t.size());
+    MapB u;
+    t.swap(u);
+    EXPECT_EQ(0, t.size());
+    EXPECT_EQ(1, u.size());
+    EXPECT_TRUE(t.find(0) == t.end());
+    EXPECT_TRUE(u[0] == 1);
+}
+
+
 TEST(THIS_TEST_NAME, IfContains) {
     // ----------------
     // test if_contains