diff --git a/.travis.yml b/.travis.yml
index aa7bafc0..3267434b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -51,7 +51,7 @@ before_script:
   - if [ "$CI_BUILD" = "autotools" ]; then autoreconf -i; fi
   - git submodule update --init
   - if [ "$CI_BUILD" = "autotools" ]; then ./configure --enable-werror --with-mruby --with-neverbleed LIBSPDYLAY_CFLAGS="-I$SPDYLAY_HOME/lib/includes" LIBSPDYLAY_LIBS="-L$SPDYLAY_HOME/lib/.libs -lspdylay"; fi
-  - if [ "$CI_BUILD" = "cmake" ]; then cmake -DENABLE_WERROR=1 -DWITH_MRUBY=1 -DWITH_NEVERBLEED=1 -DSPDYLAY_INCLUDE_DIR="$SPDYLAY_HOME/lib/includes" -DSPDYLAY_LIBRARY="$SPDYLAY_HOME/lib/.libs/libspdylay.so"; fi
+  - if [ "$CI_BUILD" = "cmake" ]; then cmake -DCMAKE_BUILD_TYPE=None -DENABLE_WERROR=1 -DWITH_MRUBY=1 -DWITH_NEVERBLEED=1 -DSPDYLAY_INCLUDE_DIR="$SPDYLAY_HOME/lib/includes" -DSPDYLAY_LIBRARY="$SPDYLAY_HOME/lib/.libs/libspdylay.so"; fi
 script:
   - make
   - make check
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c7a0f4c0..29277fac 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -40,6 +40,17 @@ set(LT_VERSION "${LT_SOVERSION}.${LT_AGE}.${LT_REVISION}")
 set(PACKAGE_VERSION     "${PROJECT_VERSION}")
 HexVersion(PACKAGE_VERSION_NUM ${PROJECT_VERSION_MAJOR} ${PROJECT_VERSION_MINOR} ${PROJECT_VERSION_PATCH})
 
+if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
+  # Note that RelWithDebInfo disables assertions, use Debug or None to keep them.
+  set(CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "Choose the build type" FORCE)
+
+  # Include "None" as option to disable any additional (optimization) flags,
+  # relying on just CMAKE_C_FLAGS and CMAKE_CXX_FLAGS (which are empty by
+  # default). These strings are presented in cmake-gui.
+  set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS
+    "None" "Debug" "Release" "MinSizeRel" "RelWithDebInfo")
+endif()
+
 include(GNUInstallDirs)
 
 # For Python bindings and documentation
@@ -227,7 +238,6 @@ else()
 endif()
 
 # Checks for header files.
-# XXX AC_HEADER_ASSERT adds --disable-assert which sets -DNDEBUG
 include(CheckIncludeFile)
 check_include_file("arpa/inet.h"    HAVE_ARPA_INET_H)
 check_include_file("fcntl.h"        HAVE_FCNTL_H)