Index: share/mk/bsd.obj.mk
===================================================================
--- share/mk/bsd.obj.mk
+++ share/mk/bsd.obj.mk
@@ -51,6 +51,8 @@
 .if defined(NO_OBJ)
 # but this makefile does not want it!
 .OBJDIR: ${.CURDIR}
+.elif ${.OBJDIR} == ${.CURDIR}
+.error .OBJDIR not properly set by auto.obj.mk.
 .endif
 # Handle special case where SRCS is full-pathed and requires
 # nested objdirs.  This duplicates some auto.obj.mk logic.
@@ -85,6 +87,10 @@
 CANONICALOBJDIR:=/usr/obj${.CURDIR}
 .endif
 
+.if ${CANONICALOBJDIR} == /${RELDIR} || ${.OBJDIR} == /${RELDIR}
+.error .OBJDIR incorrectly set to /${RELDIR}
+.endif
+
 OBJTOP?= ${.OBJDIR:S,${.CURDIR},,}${SRCTOP}
 
 #
Index: share/mk/src.sys.obj.mk
===================================================================
--- share/mk/src.sys.obj.mk
+++ share/mk/src.sys.obj.mk
@@ -157,9 +157,8 @@
 .export MK_AUTO_OBJ
 .endif	# ${MK_AUTO_OBJ} == "no" && ...
 
-# Assign this directory as .OBJDIR if possible after determining if AUTO_OBJ
-# can be enabled by default.
-.if ${MK_AUTO_OBJ} == "no"
+# Assign this directory as .OBJDIR if possible.
+#
 # The expected OBJDIR already exists, set it as .OBJDIR.
 .if !empty(MAKEOBJDIRPREFIX) && exists(${MAKEOBJDIRPREFIX}${.CURDIR})
 .OBJDIR: ${MAKEOBJDIRPREFIX}${.CURDIR}
@@ -172,5 +171,11 @@
 .elif ${MAKE_VERSION} <= 20170720 && \
     ${.CURDIR} == ${SRCTOP} && ${.OBJDIR} == ${SRCTOP}/
 .OBJDIR: ${.CURDIR}
+.else
+# The OBJDIR we wanted does not yet exist, ensure we default to safe .CURDIR
+# in case make started with a bogus MAKEOBJDIR, that expanded before OBJTOP
+# was set, that happened to match some unexpected directory.
+# Either auto.obj.mk or bsd.obj.mk will create the directory and fix .OBJDIR
+# later.
+.OBJDIR: ${.CURDIR}
 .endif
-.endif	# ${MK_AUTO_OBJ} == "no"
Index: share/mk/sys.mk
===================================================================
--- share/mk/sys.mk
+++ share/mk/sys.mk
@@ -125,6 +125,9 @@
 .if defined(__objdir) && ${.OBJDIR} != ${__objdir}
 .MAKEOVERRIDES+= MK_AUTO_OBJ
 MK_AUTO_OBJ=	no
+# Fallback to safe .CURDIR rather than whatever make happened
+# to start with.  bsd.obj.mk will fix it later.
+.OBJDIR: ${.CURDIR}
 .endif
 .endif
 .endif	# ${MK_AUTO_OBJ} == "yes"