diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9dafac9..7f1e51f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -15,7 +15,9 @@ include(CheckCXXCompilerFlag)
 if("${CMAKE_VERSION}" VERSION_GREATER 3.1)
     set(CMAKE_CXX_EXTENSIONS OFF CACHE BOOL "Boolean specifying whether compiler specific extensions are requested.")
     if(NOT DEFINED CMAKE_CXX_STANDARD)
-        set(CMAKE_CXX_STANDARD 11 CACHE STRING "The C++ standard whose features are requested to build all targets.")
+        message(FATAL_ERROR "CMAKE_CXX_STANDARD is not defined. \
+The C++ standard whose features are requested to *build* all targets. \
+Remember that toml11 is a header only library that does NOT require compilation to use.")
     endif()
     set(CMAKE_CXX_STANDARD_REQUIRED ON CACHE BOOL "Boolean describing whether the value of CXX_STANDARD is a requirement.")
 else()
diff --git a/appveyor.yml b/appveyor.yml
index 9fc6a73..3871566 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -17,7 +17,7 @@ build_script:
   - cd C:\toml11
   - mkdir build
   - cd build
-  - cmake -G"%generator%" -DBOOST_ROOT=C:/Libraries/boost_1_69_0 -Dtoml11_BUILD_TEST=ON ..
+  - cmake -G"%generator%" -DCMAKE_CXX_STANDARD=11 -DBOOST_ROOT=C:/Libraries/boost_1_69_0 -Dtoml11_BUILD_TEST=ON ..
   - cmake --build . --config "%configuration%"
   - file --mime-encoding tests/toml/tests/hard_example_unicode.toml