diff --git a/CMakeLists.txt b/CMakeLists.txt
index edea9441..a426bbde 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -185,7 +185,7 @@ if ( CMAKE_COMPILER_IS_GNUCC OR CMAKE_C_COMPILER_ID STREQUAL "Clang" OR CMAKE_C_
   endif ( NOT APPLE AND NOT OS2 )
   
   # define some warning flags
-  set ( CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -W -Wpointer-arith -Wcast-qual -Wstrict-prototypes -Wno-unused-parameter -Wdeclaration-after-statement" )
+  set ( CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -W -Wpointer-arith -Wcast-qual -Wstrict-prototypes -Wno-unused-parameter -Wdeclaration-after-statement -Werror=implicit-function-declaration" )
   
   # prepend to build type specific flags, to allow users to override
   set ( CMAKE_C_FLAGS_DEBUG "-g ${CMAKE_C_FLAGS_DEBUG}" )