diff --git a/Makefile.inc1 b/Makefile.inc1
index 2f58ac495411..b6fbfb2dedfb 100644
--- a/Makefile.inc1
+++ b/Makefile.inc1
@@ -738,6 +738,10 @@ update:
 # legacy: Build compatibility shims for the next three targets
 #
 legacy:
+.if ${BOOTSTRAPPING} < 503000
+	@echo "ERROR: Source upgrades from versions prior to 5.3 not supported."; \
+	false
+.endif
 .for _tool in tools/build
 	${_+_}@${ECHODIR} "===> ${_tool} (obj,includes,depend,all,install)"; \
 	    cd ${.CURDIR}/${_tool}; \