diff --git a/makefiles/cflags.inc.mk b/makefiles/cflags.inc.mk
index b35ddc98c58c0ede60a42fda2022a14a55ddadf2..34f878b79bf6a6957672efb65afe85bb5c13e905 100644
--- a/makefiles/cflags.inc.mk
+++ b/makefiles/cflags.inc.mk
@@ -60,6 +60,9 @@ CFLAGS += -fno-common
 # Enable all default warnings
 CFLAGS += -Wall
 
+# Warn if a user-supplied include directory does not exist.
+CFLAGS += -Wmissing-include-dirs
+
 ifeq (,$(filter -DDEVELHELP,$(CFLAGS)))
   ifneq (1,$(FORCE_ASSERTS))
     CFLAGS += -DNDEBUG