diff --git a/doc/container.qbk b/doc/container.qbk
index 0ab4a5c..34fb069 100644
--- a/doc/container.qbk
+++ b/doc/container.qbk
@@ -1247,6 +1247,7 @@ use [*Boost.Container]? There are several reasons for that:
    * [@https://github.com/boostorg/container/pull/110 GitHub #110: ['"Avoid gcc 9 deprecated copy warnings in new_allocator.hpp"]].
    * [@https://github.com/boostorg/container/issues/112 GitHub #112: ['"vector::resize() compilation error with msvc-10..12: data is not a member of boost::detail::aligned_storage"]].
    * [@https://github.com/boostorg/container/issues/117 GitHub #117: ['"flat_map/map::insert_or_assign with hint has wrong return types"]].
+   * [@https://github.com/boostorg/container/issues/118 GitHub #118: ['"Non-unique inplace_set_difference used in in flat_tree_merge_unique and iterator invalidation in insert_unique"]].
 
 [endsect]
 
diff --git a/include/boost/container/detail/flat_tree.hpp b/include/boost/container/detail/flat_tree.hpp
index c1a5b34..4d86f9e 100644
--- a/include/boost/container/detail/flat_tree.hpp
+++ b/include/boost/container/detail/flat_tree.hpp
@@ -205,7 +205,7 @@ BOOST_CONTAINER_FORCEINLINE void flat_tree_merge_unique  //has_merge_unique == f
 
    size_type const old_sz = dest.size();
    iterator const first_new = dest.insert(dest.cend(), first, last );
-   iterator e = boost::movelib::inplace_set_difference(first_new, dest.end(), dest.begin(), first_new, comp);
+   iterator e = boost::movelib::inplace_set_unique_difference(first_new, dest.end(), dest.begin(), first_new, comp);
    dest.erase(e, dest.end());
    dtl::bool_<is_contiguous_container<SequenceContainer>::value> contiguous_tag;
    (flat_tree_container_inplace_merge)(dest, dest.begin()+old_sz, comp, contiguous_tag);
@@ -883,10 +883,14 @@ class flat_tree
       //Step 3: only left unique values from the back not already present in the original range
       typename container_type::iterator const e = boost::movelib::inplace_set_unique_difference
          (it, seq.end(), seq.begin(), it, val_cmp);
-      seq.erase(e, seq.cend());
 
-      //Step 4: merge both ranges
-      (flat_tree_container_inplace_merge)(seq, it, this->priv_value_comp(), contiguous_tag);
+      seq.erase(e, seq.cend());
+      //it might be invalidated by erasing [e, seq.end) if e == it
+      if (it != e)
+      {
+         //Step 4: merge both ranges
+         (flat_tree_container_inplace_merge)(seq, it, this->priv_value_comp(), contiguous_tag);
+      }
    }
 
    template <class InIt>
diff --git a/test/flat_map_adaptor_test.cpp b/test/flat_map_adaptor_test.cpp
new file mode 100644
index 0000000..520ef3c
--- /dev/null
+++ b/test/flat_map_adaptor_test.cpp
@@ -0,0 +1,104 @@
+//////////////////////////////////////////////////////////////////////////////
+//
+// (C) Copyright Ion Gaztanaga 2004-2019. Distributed under the Boost
+// Software License, Version 1.0. (See accompanying file
+// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/libs/container for documentation.
+//
+//////////////////////////////////////////////////////////////////////////////
+#include <boost/container/flat_map.hpp>
+#include <boost/container/small_vector.hpp>
+#include <boost/container/static_vector.hpp>
+#include <boost/container/stable_vector.hpp>
+#include <boost/container/vector.hpp>
+#include <boost/container/deque.hpp>
+
+#include <boost/container/detail/container_or_allocator_rebind.hpp>
+
+#include "map_test.hpp"
+#include <map>
+
+using namespace boost::container;
+
+template<class VoidAllocatorOrContainer>
+struct GetMapContainer
+{
+   template<class ValueType>
+   struct apply
+   {
+      typedef std::pair<ValueType, ValueType> type_t;
+      typedef flat_map< ValueType
+                 , ValueType
+                 , std::less<ValueType>
+                 , typename boost::container::dtl::container_or_allocator_rebind<VoidAllocatorOrContainer, type_t>::type
+                 > map_type;
+
+      typedef flat_multimap< ValueType
+                 , ValueType
+                 , std::less<ValueType>
+                 , typename boost::container::dtl::container_or_allocator_rebind<VoidAllocatorOrContainer, type_t>::type
+                 > multimap_type;
+   };
+};
+
+int main()
+{
+   using namespace boost::container::test;
+
+   ////////////////////////////////////
+   //    Testing sequence container implementations
+   ////////////////////////////////////
+   {
+      typedef std::map<int, int>                                     MyStdMap;
+      typedef std::multimap<int, int>                                MyStdMultiMap;
+
+      if (0 != test::map_test
+         < GetMapContainer<vector<std::pair<int, int> > >::apply<int>::map_type
+         , MyStdMap
+         , GetMapContainer<vector<std::pair<int, int> > >::apply<int>::multimap_type
+         , MyStdMultiMap>()) {
+         std::cout << "Error in map_test<vector<std::pair<int, int> > >" << std::endl;
+         return 1;
+      }
+
+      if (0 != test::map_test
+         < GetMapContainer<small_vector<std::pair<int, int>, 7> >::apply<int>::map_type
+         , MyStdMap
+         , GetMapContainer<small_vector<std::pair<int, int>, 7> >::apply<int>::multimap_type
+         , MyStdMultiMap>()) {
+         std::cout << "Error in map_test<small_vector<std::pair<int, int>, 7> >" << std::endl;
+         return 1;
+      }
+
+       if (0 != test::map_test
+         < GetMapContainer<static_vector<std::pair<int, int>, MaxElem * 10> >::apply<int>::map_type
+         , MyStdMap
+         , GetMapContainer<static_vector<std::pair<int, int>, MaxElem * 10> >::apply<int>::multimap_type
+         , MyStdMultiMap>()) {
+         std::cout << "Error in map_test<static_vector<std::pair<int, int>, MaxElem * 10> >" << std::endl;
+         return 1;
+      }
+
+      if (0 != test::map_test
+         < GetMapContainer<stable_vector<std::pair<int, int> > >::apply<int>::map_type
+         , MyStdMap
+         , GetMapContainer<stable_vector<std::pair<int, int> > >::apply<int>::multimap_type
+         , MyStdMultiMap>()) {
+         std::cout << "Error in map_test<stable_vector<std::pair<int, int> > >" << std::endl;
+         return 1;
+      }
+
+
+      if (0 != test::map_test
+         < GetMapContainer<deque<std::pair<int, int> > >::apply<int>::map_type
+         , MyStdMap
+         , GetMapContainer<deque<std::pair<int, int> > >::apply<int>::multimap_type
+         , MyStdMultiMap>()) {
+         std::cout << "Error in map_test<deque<std::pair<int, int> > >" << std::endl;
+         return 1;
+      }
+   }
+
+   return 0;
+}
diff --git a/test/flat_set_adaptor_test.cpp b/test/flat_set_adaptor_test.cpp
new file mode 100644
index 0000000..b59ab17
--- /dev/null
+++ b/test/flat_set_adaptor_test.cpp
@@ -0,0 +1,101 @@
+//////////////////////////////////////////////////////////////////////////////
+//
+// (C) Copyright Ion Gaztanaga 2004-2019. Distributed under the Boost
+// Software License, Version 1.0. (See accompanying file
+// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// See http://www.boost.org/libs/container for documentation.
+//
+//////////////////////////////////////////////////////////////////////////////
+#include <boost/container/flat_set.hpp>
+#include <boost/container/small_vector.hpp>
+#include <boost/container/static_vector.hpp>
+#include <boost/container/stable_vector.hpp>
+#include <boost/container/vector.hpp>
+#include <boost/container/deque.hpp>
+
+#include <boost/container/detail/container_or_allocator_rebind.hpp>
+
+#include "set_test.hpp"
+#include <set>
+
+using namespace boost::container;
+
+template<class VoidAllocatorOrContainer>
+struct GetSetContainer
+{
+   template<class ValueType>
+   struct apply
+   {
+      typedef flat_set < ValueType
+                       , std::less<ValueType>
+                       , typename boost::container::dtl::container_or_allocator_rebind<VoidAllocatorOrContainer, ValueType>::type
+                        > set_type;
+
+      typedef flat_multiset < ValueType
+                            , std::less<ValueType>
+                            , typename boost::container::dtl::container_or_allocator_rebind<VoidAllocatorOrContainer, ValueType>::type
+                            > multiset_type;
+   };
+};
+
+int main()
+{
+   using namespace boost::container::test;
+
+   ////////////////////////////////////
+   //    Testing sequence container implementations
+   ////////////////////////////////////
+   {
+      typedef std::set<int>                                     MyStdSet;
+      typedef std::multiset<int>                                MyStdMultiSet;
+
+      if (0 != test::set_test
+         < GetSetContainer<vector<int> >::apply<int>::set_type
+         , MyStdSet
+         , GetSetContainer<vector<int> >::apply<int>::multiset_type
+         , MyStdMultiSet>()) {
+         std::cout << "Error in set_test<vector<int> >" << std::endl;
+         return 1;
+      }
+
+      if (0 != test::set_test
+         < GetSetContainer<small_vector<int, 7> >::apply<int>::set_type
+         , MyStdSet
+         , GetSetContainer<small_vector<int, 7> >::apply<int>::multiset_type
+         , MyStdMultiSet>()) {
+         std::cout << "Error in set_test<small_vector<int, 7> >" << std::endl;
+         return 1;
+      }
+
+       if (0 != test::set_test
+         < GetSetContainer<static_vector<int, MaxElem * 10> >::apply<int>::set_type
+         , MyStdSet
+         , GetSetContainer<static_vector<int, MaxElem * 10> >::apply<int>::multiset_type
+         , MyStdMultiSet>()) {
+         std::cout << "Error in set_test<static_vector<int, MaxElem * 10> >" << std::endl;
+         return 1;
+      }
+
+      if (0 != test::set_test
+         < GetSetContainer<stable_vector<int> >::apply<int>::set_type
+         , MyStdSet
+         , GetSetContainer<stable_vector<int> >::apply<int>::multiset_type
+         , MyStdMultiSet>()) {
+         std::cout << "Error in set_test<stable_vector<int> >" << std::endl;
+         return 1;
+      }
+
+
+      if (0 != test::set_test
+         < GetSetContainer<deque<int> >::apply<int>::set_type
+         , MyStdSet
+         , GetSetContainer<deque<int> >::apply<int>::multiset_type
+         , MyStdMultiSet>()) {
+         std::cout << "Error in set_test<deque<int> >" << std::endl;
+         return 1;
+      }
+   }
+
+   return 0;
+}
diff --git a/test/map_test.hpp b/test/map_test.hpp
index 99bd42d..160c46a 100644
--- a/test/map_test.hpp
+++ b/test/map_test.hpp
@@ -14,6 +14,7 @@
 #include <boost/container/detail/config_begin.hpp>
 #include "check_equal_containers.hpp"
 #include "print_container.hpp"
+#include "movable_int.hpp"
 #include <boost/container/detail/pair.hpp>
 #include <boost/move/iterator.hpp>
 #include <boost/move/utility_core.hpp>
diff --git a/test/set_test.hpp b/test/set_test.hpp
index c31882a..7bc3a09 100644
--- a/test/set_test.hpp
+++ b/test/set_test.hpp
@@ -14,6 +14,7 @@
 #include <boost/container/detail/config_begin.hpp>
 #include "check_equal_containers.hpp"
 #include "print_container.hpp"
+#include "movable_int.hpp"
 #include <boost/move/utility_core.hpp>
 #include <boost/move/iterator.hpp>
 #include <boost/move/make_unique.hpp>