diff --git a/CMakeLists.txt b/CMakeLists.txt
index fee3b6b3..0478b352 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,7 +41,9 @@ if(DEV_MODE)
                           -Werror=declaration-missing-parameter-type
                           -Werror=switch
                           -Werror=return-type
-                          -W4)
+                          -W4
+                          -WX4013 # undefined; assuming extern returning int
+                          )
 endif()
 
 add_subdirectory("corec")