diff --git a/Makefile.inc1 b/Makefile.inc1
index 06d6951c91d2..0fd7e240d133 100644
--- a/Makefile.inc1
+++ b/Makefile.inc1
@@ -28,6 +28,17 @@
 # /usr/share/mk.  These include:
 #		obj depend all install clean cleandepend cleanobj
 
+# You are supposed to define both of these when calling Makefile.inc1
+# directly.  However, some old scripts don't.  Cope for the moment, but
+# issue a new warning for a transition period.
+.if defined(TARGET) && !defined(TARGET_ARCH)
+.warning "You must pass both TARGET and TARGET_ARCH to Makefile.inc1.  Setting TARGET_ARCH=${TARGET}."
+TARGET_ARCH=${TARGET}
+.endif
+.if !defined(TARGET) || !defined(TARGET_ARCH)
+.error "Both TARGET and TARGET_ARCH must be defined."
+.endif
+
 .include <bsd.own.mk>
 .include <bsd.arch.inc.mk>