diff --git a/etc/defaults/make.conf b/etc/defaults/make.conf
index 8f4ba6f0c612..54cdca9c261d 100644
--- a/etc/defaults/make.conf
+++ b/etc/defaults/make.conf
@@ -50,7 +50,7 @@
 # putting "CFLAGS+=${BDECFLAGS}" in /etc/make.conf.
 #
 BDECFLAGS=	-W -Wall -ansi -pedantic -Wbad-function-cast -Wcast-align \
-		-Wcast-qual -Wchar-subscripts -Wconversion -Winline \
+		-Wcast-qual -Wchar-subscripts -Wconversion -Winline -Wundef \
 		-Wmissing-prototypes -Wnested-externs -Wpointer-arith \
 		-Wredundant-decls -Wshadow -Wstrict-prototypes -Wwrite-strings
 #
diff --git a/share/examples/etc/make.conf b/share/examples/etc/make.conf
index 8f4ba6f0c612..54cdca9c261d 100644
--- a/share/examples/etc/make.conf
+++ b/share/examples/etc/make.conf
@@ -50,7 +50,7 @@
 # putting "CFLAGS+=${BDECFLAGS}" in /etc/make.conf.
 #
 BDECFLAGS=	-W -Wall -ansi -pedantic -Wbad-function-cast -Wcast-align \
-		-Wcast-qual -Wchar-subscripts -Wconversion -Winline \
+		-Wcast-qual -Wchar-subscripts -Wconversion -Winline -Wundef \
 		-Wmissing-prototypes -Wnested-externs -Wpointer-arith \
 		-Wredundant-decls -Wshadow -Wstrict-prototypes -Wwrite-strings
 #