Index: Makefile
===================================================================
--- Makefile
+++ Makefile
@@ -143,13 +143,6 @@
 
 PATH=	/sbin:/bin:/usr/sbin:/usr/bin
 MAKEOBJDIRPREFIX?=	/usr/obj
-_MAKEOBJDIRPREFIX!= /usr/bin/env -i PATH=${PATH} ${MAKE} \
-    ${.MAKEFLAGS:MMAKEOBJDIRPREFIX=*} __MAKE_CONF=${__MAKE_CONF} \
-    -f /dev/null -V MAKEOBJDIRPREFIX dummy
-.if !empty(_MAKEOBJDIRPREFIX)
-.error MAKEOBJDIRPREFIX can only be set in environment, not as a global\
-	(in make.conf(5)) or command-line variable.
-.endif
 
 # We often need to use the tree's version of make to build it.
 # Choices add to complexity though.
Index: share/mk/sys.mk
===================================================================
--- share/mk/sys.mk
+++ share/mk/sys.mk
@@ -33,6 +33,13 @@
 	${__DEFAULT_YES_OPTIONS} \
 	${__DEFAULT_DEPENDENT_OPTIONS:H}
 
+__ENV_ONLY_VARS= \
+	MAKEOBJDIR \
+	MAKEOBJDIRPREFIX
+.for _var in ${__ENV_ONLY_VARS}
+_pre_includes_${_var:tl}:=	${${_var}:U__null}
+.endfor
+
 # early include for customization
 # see local.sys.mk below
 # Not included when building in fmake compatibility mode (still needed
@@ -50,6 +57,9 @@
 .endif
 .endif
 .if ${MK_AUTO_OBJ} == "yes"
+# Reset, since it is allowed to be set from src-env.conf included before this.
+_pre_includes_makeobjdirprefix:= ${MAKEOBJDIRPREFIX:U__null}
+_pre_includes_makeobjdir:= ${MAKEOBJDIR:U__null}
 # This needs to be done early - before .PATH is computed
 # Don't do this for 'make showconfig' as it enables all options where meta mode
 # is not expected.
@@ -406,6 +416,15 @@
 	path=${__MAKE_SHELL}
 .endif
 
+# Ensure MAKEOBJDIRPREFIX was not incorrectly set.
+.for _var in ${__ENV_ONLY_VARS}
+.if ${.MAKEOVERRIDES:M${_var}} || (defined(${_var}) && \
+    ${${_var}} != ${_pre_includes_${_var:tl}})
+.error ${_var} can only be set in environment, not as a global (in make.conf(5)) or command-line variable.
+.endif
+.undef _pre_includes_${_var:tl}
+.endfor
+
 # Hack for ports compatibility. Historically, ports makefiles have
 # assumed they can examine MACHINE_CPU without including anything
 # because this was automatically included in sys.mk. For /usr/src,