diff --git a/reactos/cmake/compilerflags.cmake b/reactos/cmake/compilerflags.cmake
index bc48e908197..d6928a19103 100644
--- a/reactos/cmake/compilerflags.cmake
+++ b/reactos/cmake/compilerflags.cmake
@@ -4,8 +4,18 @@ macro(add_compiler_flags)
     foreach(flag ${ARGN})
         set(flags_list "${flags_list} ${flag}")
     endforeach()
-    set(CMAKE_C_FLAGS ${flags_list})
-    set(CMAKE_CXX_FLAGS ${flags_list})
+
+    if(CMAKE_C_FLAGS)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${flags_list}")
+    else()
+        set(CMAKE_C_FLAGS ${flags_list})
+    endif()
+
+    if(CMAKE_CXX_FLAGS)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${flags_list}")
+    else()
+        set(CMAKE_CXX_FLAGS ${flags_list})
+    endif()
 endmacro()
 
 macro(add_linkerflag MODULE _flag)