diff --git a/.travis.yml b/.travis.yml
index 3542fc3..5ff0c21 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,7 +5,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: gcc
-    env: COMPILER="g++-5" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="g++-5" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -17,7 +17,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: gcc
-    env: COMPILER="g++-6" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="g++-6" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -29,7 +29,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: gcc
-    env: COMPILER="g++-7" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="g++-7" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -41,7 +41,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: gcc
-    env: COMPILER="g++-8" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="g++-8" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -65,7 +65,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: gcc
-    env: COMPILER="g++-8" CXX_STANDARD=17 TOML_HEAD=OFF
+    env: COMPILER="g++-8" CXX_STANDARD=17
     addons:
       apt:
         sources:
@@ -89,7 +89,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: clang
-    env: COMPILER="clang++-3.9" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="clang++-3.9" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -103,7 +103,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: clang
-    env: COMPILER="clang++-4.0" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="clang++-4.0" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -117,7 +117,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: clang
-    env: COMPILER="clang++-5.0" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="clang++-5.0" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -131,7 +131,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: clang
-    env: COMPILER="clang++-6.0" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="clang++-6.0" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -145,7 +145,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: clang
-    env: COMPILER="clang++-7" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="clang++-7" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -159,7 +159,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: clang
-    env: COMPILER="clang++-8" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="clang++-8" CXX_STANDARD=11
     addons:
       apt:
         sources:
@@ -187,7 +187,7 @@ matrix:
   - os: linux
     language: cpp
     compiler: clang
-    env: COMPILER="clang++-8" CXX_STANDARD=17 TOML_HEAD=OFF
+    env: COMPILER="clang++-8" CXX_STANDARD=17
     addons:
       apt:
         sources:
@@ -212,6 +212,34 @@ matrix:
         - clang-8
         - g++-8
         - boost1.70
+  - os: linux
+    language: cpp
+    compiler: clang
+    env: COMPILER="clang++-8" CXX_STANDARD=11 WITH_ASAN=ON
+    addons:
+      apt:
+        sources:
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
+        - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-8
+        packages:
+        - clang-8
+        - g++-8
+        - boost1.70
+  - os: linux
+    language: cpp
+    compiler: clang
+    env: COMPILER="clang++-8" CXX_STANDARD=11 WITH_UBSAN=ON
+    addons:
+      apt:
+        sources:
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
+        - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-8
+        packages:
+        - clang-8
+        - g++-8
+        - boost1.70
   - os: osx
     language: cpp
     compiler: clang
@@ -234,11 +262,26 @@ script:
     tar xf cmake-3.14.5-Linux-x86_64.tar.gz -C cmake --strip-components=1
     export PATH=${TRAVIS_BUILD_DIR}/cmake/bin:${PATH}
   fi
+- |
+  if [[ "${TOML_HEAD}" != "ON" ]]; then
+    export TOML_HEAD="OFF"
+  fi
+- echo "TOML_HEAD = ${TOML_HEAD}"
+- |
+  if [[ "${WITH_ASAN}" != "ON" ]]; then
+    export WITH_ASAN="OFF"
+  fi
+- echo "WITH_ASAN = ${WITH_ASAN}"
+- |
+  if [[ "${WITH_UBSAN}" != "ON" ]]; then
+    export WITH_UBSAN="OFF"
+  fi
+- echo "WITH_UBSAN = ${WITH_UBSAN}"
 - cmake --version
 - mkdir build
 - cd build
 - git clone https://github.com/toml-lang/toml.git
-- cmake -DCMAKE_CXX_COMPILER=$COMPILER -DCMAKE_CXX_STANDARD=$CXX_STANDARD -DTOML11_USE_UNRELEASED_TOML_FEATURES=${TOML_HEAD} ..
+- cmake -DCMAKE_CXX_COMPILER=$COMPILER -DCMAKE_CXX_STANDARD=$CXX_STANDARD -DTOML11_USE_UNRELEASED_TOML_FEATURES=${TOML_HEAD} -Dtoml11_TEST_WITH_ASAN=${WITH_ASAN} -Dtoml11_TEST_WITH_UBSAN=${WITH_UBSAN} ..
 - make
 - ctest --output-on-failure
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
index e0315eb..f75ffcc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -11,6 +11,8 @@ set(toml11_VERSION
 )
 
 option(toml11_BUILD_TEST "Build toml tests" ON)
+option(toml11_TEST_WITH_ASAN  "use LLVM address sanitizer" OFF)
+option(toml11_TEST_WITH_UBSAN "use LLVM undefined behavior sanitizer" OFF)
 
 include(CheckCXXCompilerFlag)
 if("${CMAKE_VERSION}" VERSION_GREATER 3.1)
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index e527e04..8570aeb 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -104,6 +104,19 @@ foreach(TEST_NAME ${TEST_NAMES})
     add_executable(${TEST_NAME} ${TEST_NAME}.cpp)
     target_link_libraries(${TEST_NAME} ${Boost_UNIT_TEST_FRAMEWORK_LIBRARY} toml11::toml11)
     target_include_directories(${TEST_NAME} PRIVATE ${Boost_INCLUDE_DIRS})
+
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+        if(toml11_TEST_WITH_ASAN)
+            set_target_properties(${TEST_NAME} PROPERTIES
+                COMPILE_FLAGS "-fsanitize=address -fno-omit-frame-pointer"
+                LINK_FLAGS    "-fsanitize=address -fno-omit-frame-pointer")
+        elseif(toml11_TEST_WITH_UBSAN)
+            set_target_properties(${TEST_NAME} PROPERTIES
+                COMPILE_FLAGS "-fsanitize=undefined"
+                LINK_FLAGS    "-fsanitize=undefined")
+        endif()
+    endif()
+
     add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME} WORKING_DIRECTORY ${PROJECT_BINARY_DIR})
 
     # Set the PATH to be able to find Boost DLL
diff --git a/tests/test_parse_inline_table.cpp b/tests/test_parse_inline_table.cpp
index e1934a5..5fa99c1 100644
--- a/tests/test_parse_inline_table.cpp
+++ b/tests/test_parse_inline_table.cpp
@@ -46,3 +46,19 @@ BOOST_AUTO_TEST_CASE(test_inline_table_value)
         TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, "{type.name = \"pug\"}", value(t));
     }
 }
+
+BOOST_AUTO_TEST_CASE(test_inline_table_immutability)
+{
+    {
+        std::istringstream stream(std::string(
+                "a = {b = 1}\n"
+                "a.c = 2\n"));
+        BOOST_CHECK_THROW(toml::parse(stream), toml::syntax_error);
+    }
+    {
+        std::istringstream stream(std::string(
+                "a = {b = {c = 1}}\n"
+                "a.b.d = 2\n"));
+        BOOST_CHECK_THROW(toml::parse(stream), toml::syntax_error);
+    }
+}
diff --git a/toml/parser.hpp b/toml/parser.hpp
index 9e511ad..7cf644a 100644
--- a/toml/parser.hpp
+++ b/toml/parser.hpp
@@ -1331,7 +1331,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
             tab->insert(std::make_pair(k, v));
             return ok(true);
         }
-        else
+        else // k is not the last one, we should insert recursively
         {
             // if there is no corresponding value, insert it first.
             // related: you don't need to write
@@ -1347,6 +1347,27 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
             // type checking...
             if(tab->at(k).is_table())
             {
+                // According to toml-lang/toml:36d3091b3 "Clarify that inline
+                // tables are immutable", check if it adds key-value pair to an
+                // inline table.
+                //   This is one of the unreleased (after-0.5.0) toml feature.
+                // But this is marked as "Clarify", so TOML-lang intended that
+                // inline tables are immutable in all version.
+                {
+                    // here, if the value is a (multi-line) table, the region
+                    // should be something like `[table-name]`.
+                    if(get_region(tab->at(k)).front() == '{')
+                    {
+                        throw syntax_error(format_underline(concat_to_string(
+                            "toml::insert_value: inserting to an inline table (",
+                            format_dotted_keys(first, std::next(iter)),
+                            ") but inline tables are immutable"), {
+                                {std::addressof(get_region(tab->at(k))),
+                                 "inline tables are immutable"},
+                                {std::addressof(get_region(v)), "inserting this"}
+                            }), v.location());
+                    }
+                }
                 tab = std::addressof((*tab)[k].as_table());
             }
             else if(tab->at(k).is_array()) // inserting to array-of-tables?
diff --git a/toml/region.hpp b/toml/region.hpp
index 973bf48..bf9ba12 100644
--- a/toml/region.hpp
+++ b/toml/region.hpp
@@ -41,6 +41,7 @@ struct region_base
     region_base& operator=(region_base&&     ) = default;
 
     virtual bool is_ok()           const noexcept {return false;}
+    virtual char front()           const noexcept {return '\0';}
 
     virtual std::string str()      const {return std::string("unknown region");}
     virtual std::string name()     const {return std::string("unknown file");}
@@ -89,6 +90,7 @@ struct location final : public region_base
     ~location() = default;
 
     bool is_ok() const noexcept override {return static_cast<bool>(source_);}
+    char front() const noexcept override {return *iter_;}
 
     // this const prohibits codes like `++(loc.iter())`.
     const const_iterator iter()  const noexcept {return iter_;}
@@ -240,6 +242,7 @@ struct region final : public region_base
     }
 
     bool is_ok() const noexcept override {return static_cast<bool>(source_);}
+    char front() const noexcept override {return *first_;}
 
     std::string str()  const override {return make_string(first_, last_);}
     std::string line() const override