diff --git a/CMakeLists.txt b/CMakeLists.txt
index 93ee787..00b3b41 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 2.6)
 
 PROJECT(ubox C)
-ADD_DEFINITIONS(-Os -Wall -Werror --std=gnu99 -g3)
+ADD_DEFINITIONS(-Os -Wall -Werror --std=gnu99 -g3 -Wmissing-declarations)
 
 OPTION(BUILD_LUA "build Lua plugin" ON)