diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4fa0c31ff..ccb19ae88 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -19,7 +19,7 @@ PROJECT(weechat)
 SET(CMAKE_VERBOSE_MAKEFILE OFF)
 SET(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake ${CMAKE_MODULE_PATH})
 SET(CMAKE_SKIP_RPATH ON)
-SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -W")
+SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -W -Werror-implicit-function-declaration")
 IF(PREFIX)
   SET(CMAKE_INSTALL_PREFIX ${PREFIX} CACHE PATH "Install path prefix" FORCE)
 ENDIF(PREFIX)
diff --git a/configure.in b/configure.in
index fdd1a94af..6373b9ac5 100644
--- a/configure.in
+++ b/configure.in
@@ -818,7 +818,7 @@ AC_DEFINE_UNQUOTED(WEECHAT_SHAREDIR, "$WEECHAT_SHAREDIR")
 weechat_libdir=${libdir}/weechat
 AC_SUBST(weechat_libdir)
 
-COMMON_CFLAGS="-Wall -W"
+COMMON_CFLAGS="-Wall -W -Werror-implicit-function-declaration"
 AC_MSG_CHECKING([whether we have GNU assembler])
 GAS=`as --version < /dev/null 2>/dev/null | grep GNU`
 if test "$GAS"; then