diff --git a/CMakeLists.txt b/CMakeLists.txt
index 604cc76a..6f83f8ae 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -315,6 +315,7 @@ endif ()
 
 if (LIBZMQ_WERROR)
   zmq_check_cxx_flag_prepend ("-Werror")
+  zmq_check_cxx_flag_prepend ("/WX")
   if (NOT "${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
     zmq_check_cxx_flag_prepend ("-errwarn=%all")
   endif()
diff --git a/appveyor.yml b/appveyor.yml
index 36d27609..20ccc665 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -93,7 +93,7 @@ before_build:
   - cmd: set LIBZMQ_BUILDDIR=C:\projects\build_libzmq
   - cmd: md "%LIBZMQ_BUILDDIR%"
   - cd "%LIBZMQ_BUILDDIR%"
-  - cmd: cmake -D CMAKE_INCLUDE_PATH="%SODIUM_INCLUDE_DIR%" -D CMAKE_LIBRARY_PATH="%SODIUM_LIBRARY_DIR%" -D WITH_LIBSODIUM="%WITH_LIBSODIUM%" -D ENABLE_CURVE="%ENABLE_CURVE%" -D POLLER="%POLLER%" -D CMAKE_C_FLAGS_RELEASE="/MT" -D CMAKE_C_FLAGS_DEBUG="/MTd" -D WITH_LIBSODIUM="%WITH_LIBSODIUM%" -G "%CMAKE_GENERATOR%" "%APPVEYOR_BUILD_FOLDER%"
+  - cmd: cmake -D CMAKE_INCLUDE_PATH="%SODIUM_INCLUDE_DIR%" -D CMAKE_LIBRARY_PATH="%SODIUM_LIBRARY_DIR%" -D WITH_LIBSODIUM="%WITH_LIBSODIUM%" -D ENABLE_CURVE="%ENABLE_CURVE%" -D POLLER="%POLLER%" -D CMAKE_C_FLAGS_RELEASE="/MT" -D CMAKE_C_FLAGS_DEBUG="/MTd" -D WITH_LIBSODIUM="%WITH_LIBSODIUM%" -D LIBZMQ_WERROR="ON" -G "%CMAKE_GENERATOR%" "%APPVEYOR_BUILD_FOLDER%"
 
 build:
   parallel: true