Index: head/sys/modules/linprocfs/Makefile
===================================================================
--- head/sys/modules/linprocfs/Makefile
+++ head/sys/modules/linprocfs/Makefile
@@ -7,4 +7,8 @@
 	device_if.h bus_if.h \
 	linprocfs.c
 
+.if !defined(KERNBUILDDIR)
+.warning Building Linuxulator outside of a kernel does not make sense
+.endif
+
 .include <bsd.kmod.mk>
Index: head/sys/modules/linsysfs/Makefile
===================================================================
--- head/sys/modules/linsysfs/Makefile
+++ head/sys/modules/linsysfs/Makefile
@@ -7,4 +7,8 @@
 	device_if.h bus_if.h  pci_if.h \
 	linsysfs.c
 
+.if !defined(KERNBUILDDIR)
+.warning Building Linuxulator outside of a kernel does not make sense
+.endif
+
 .include <bsd.kmod.mk>
Index: head/sys/modules/linux/Makefile
===================================================================
--- head/sys/modules/linux/Makefile
+++ head/sys/modules/linux/Makefile
@@ -80,4 +80,8 @@
 linux${SFX}_genassym.o: offset.inc
 	${CC} -c ${CFLAGS:N-flto:N-fno-common} ${.IMPSRC}
 
+.if !defined(KERNBUILDDIR)
+.warning Building Linuxulator outside of a kernel does not make sense
+.endif
+
 .include <bsd.kmod.mk>
Index: head/sys/modules/linux64/Makefile
===================================================================
--- head/sys/modules/linux64/Makefile
+++ head/sys/modules/linux64/Makefile
@@ -55,4 +55,8 @@
 linux_genassym.o: offset.inc
 	${CC} -c ${CFLAGS:N-flto:N-fno-common} ${.IMPSRC}
 
+.if !defined(KERNBUILDDIR)
+.warning Building Linuxulator outside of a kernel does not make sense
+.endif
+
 .include <bsd.kmod.mk>
Index: head/sys/modules/linux_common/Makefile
===================================================================
--- head/sys/modules/linux_common/Makefile
+++ head/sys/modules/linux_common/Makefile
@@ -15,4 +15,8 @@
 EXPORT_SYMS+=	linux_get_osname
 EXPORT_SYMS+=	linux_get_osrelease
 
+.if !defined(KERNBUILDDIR)
+.warning Building Linuxulator outside of a kernel does not make sense
+.endif
+
 .include <bsd.kmod.mk>