diff --git a/.travis.yml b/.travis.yml
index 1161ecb..757499f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -147,6 +147,62 @@ matrix:
             - libboost-filesystem-dev
             - libboost-thread-dev
       env: MATRIX_EVAL="CC=gcc-7 CXX=g++-7 TYPE=RelWithDebInfo STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
+    - os: linux
+      name: "GCC 8 Debug"
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-8
+            - cmake
+            - libboost-test-dev
+            - libboost-program-options-dev
+            - libboost-filesystem-dev
+            - libboost-thread-dev
+      env: MATRIX_EVAL="CC=gcc-8 CXX=g++-8 TYPE=Debug STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
+    - os: linux
+      name: "GCC 8 RelWithDebInfo"
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-8
+            - cmake
+            - libboost-test-dev
+            - libboost-program-options-dev
+            - libboost-filesystem-dev
+            - libboost-thread-dev
+      env: MATRIX_EVAL="CC=gcc-8 CXX=g++-8 TYPE=RelWithDebInfo STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
+    - os: linux
+      name: "GCC 9 Debug"
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-9
+            - cmake
+            - libboost-test-dev
+            - libboost-program-options-dev
+            - libboost-filesystem-dev
+            - libboost-thread-dev
+      env: MATRIX_EVAL="CC=gcc-9 CXX=g++-9 TYPE=Debug STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
+    - os: linux
+      name: "GCC 9 RelWithDebInfo"
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-9
+            - cmake
+            - libboost-test-dev
+            - libboost-program-options-dev
+            - libboost-filesystem-dev
+            - libboost-thread-dev
+      env: MATRIX_EVAL="CC=gcc-9 CXX=g++-9 TYPE=RelWithDebInfo STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
     - os: linux
       dist: trusty
       name: "Clang 3.6 Debug"
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ae64386..7bb5869 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -80,6 +80,11 @@ if (WSREP_LIB_MAINTAINER_MODE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
 endif()
 
+# Enable extra libstdc++ assertions with debug build.
+if (CMAKE_BUILD_TYPE STREQUAL "Debug")
+  add_definitions("-D_GLIBCXX_ASSERTIONS")
+endif()
+
 # Set up include directories
 include_directories("${CMAKE_CURRENT_SOURCE_DIR}/include")
 include_directories("${CMAKE_CURRENT_SOURCE_DIR}/wsrep-API")
diff --git a/include/wsrep/buffer.hpp b/include/wsrep/buffer.hpp
index 7eaea75..89dc822 100644
--- a/include/wsrep/buffer.hpp
+++ b/include/wsrep/buffer.hpp
@@ -85,8 +85,26 @@ namespace wsrep
         }
 
         size_t size() const { return buffer_.size(); }
-        char* data() { return &buffer_[0]; }
-        const char* data() const { return &buffer_[0]; }
+
+        /**
+         * Return pointer to underlying data array. The returned pointer
+         * may or may not be null in case of empty buffer, it is up to
+         * user to check the size of the array before dereferencing the
+         * pointer.
+         *
+         * @return Pointer to underlying data array.
+         */
+        char* data() { return buffer_.data(); }
+
+        /**
+         * Return const pointer to underlying data array. The returned pointer
+         * may or may not be null in case of empty buffer, it is up to
+         * user to check the size of the array before dereferencing the
+         * pointer.
+         *
+         * @return Const pointer to underlying data array.
+         */
+        const char* data() const { return buffer_.data(); }
 
         mutable_buffer& operator= (const mutable_buffer& other)
         {
diff --git a/src/wsrep_provider_v26.cpp b/src/wsrep_provider_v26.cpp
index 75e93ee..49ed841 100644
--- a/src/wsrep_provider_v26.cpp
+++ b/src/wsrep_provider_v26.cpp
@@ -884,7 +884,7 @@ wsrep::wsrep_provider_v26::enter_toi(
     return map_return_value(wsrep_->to_execute_start(
                                 wsrep_,
                                 client_id.get(),
-                                &wsrep_keys[0],
+                                wsrep_keys.data(),
                                 wsrep_keys.size(),
                                 &wsrep_buf,
                                 1,
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index ed667f6..74dbc94 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -7,6 +7,7 @@ add_executable(wsrep-lib_test
   mock_high_priority_service.cpp
   mock_storage_service.cpp
   test_utils.cpp
+  buffer_test.cpp
   gtid_test.cpp
   id_test.cpp
   server_context_test.cpp
diff --git a/test/buffer_test.cpp b/test/buffer_test.cpp
new file mode 100644
index 0000000..84d4a48
--- /dev/null
+++ b/test/buffer_test.cpp
@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2019 Codership Oy <info@codership.com>
+ *
+ * This file is part of wsrep-lib.
+ *
+ * Wsrep-lib is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * Wsrep-lib is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with wsrep-lib.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include "wsrep/buffer.hpp"
+#include <boost/test/unit_test.hpp>
+
+BOOST_AUTO_TEST_CASE(buffer_test_empty_access)
+{
+    wsrep::mutable_buffer buf;
+    BOOST_REQUIRE(buf.size() == 0);
+    (void)buf.data();
+}