Index: Mk/bsd.port.mk
===================================================================
--- Mk/bsd.port.mk
+++ Mk/bsd.port.mk
@@ -1196,7 +1196,8 @@
 _OSVERSION_MAJOR=	${OSVERSION:C/([0-9]?[0-9])([0-9][0-9])[0-9]{3}/\1/}
 # Sanity checks for chroot/jail building.
 # Skip if OSVERSION specified on cmdline for testing. Only works for bmake.
-.if !defined(.MAKEOVERRIDES) || !${.MAKEOVERRIDES:MOSVERSION}
+.if !defined(ALLOW_OSVERSION_MISMATCH) && \
+       (!defined(.MAKEOVERRIDES) || !${.MAKEOVERRIDES:MOSVERSION})
 .if ${_OSVERSION_MAJOR} != ${_OSRELEASE:R}
 .error UNAME_r (${_OSRELEASE}) and OSVERSION (${OSVERSION}) do not agree on major version number.
 .elif ${_OSVERSION_MAJOR} != ${OSREL:R}