diff --git a/CMakeLists.txt b/CMakeLists.txt
index 719df3e34..d0370c48d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -377,7 +377,7 @@ endif()
 
 # CXX Compiler settings
 if (CMAKE_CXX_COMPILER_ID STREQUAL GNU OR CMAKE_CXX_COMPILER_ID STREQUAL Clang)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wno-unused-parameter -std=c++98 -pedantic -Wno-long-long")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wundef -Wno-unused-parameter -std=c++98 -pedantic -Wno-long-long")
 
     if (CMAKE_CXX_COMPILER_ID STREQUAL Clang AND NOT APPLE)
         execute_process(COMMAND ${CMAKE_C_COMPILER} --version OUTPUT_VARIABLE CLANG_VERSION)