diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9de589f38..785778f40 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -32,7 +32,13 @@ MY_CHECK_CXX_COMPILER_FLAG("-std=c++20")
 IF (have_CXX__std_c__20)
   SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++20")
 ELSE()
-  SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++2a")
+  MY_CHECK_CXX_COMPILER_FLAG("-std=c++2a")
+  IF (have_CXX__std_c__2a)
+    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++2a")
+  ELSE()
+    MESSAGE_ONCE(CS_NO_CXX20 "C++ Compiler does not understand -std=c++20")
+    return()
+  ENDIF()
 ENDIF()
 
 UNSET(CMAKE_CXX_STANDARD)