diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -80,6 +80,7 @@
 		-Wno-error=alloca-larger-than=
 .if ${COMPILER_VERSION} >= 120100
 CWARNEXTRA+=	-Wno-error=nonnull				\
+		-Wnoerror=unused-variable			\
 		-Wno-dangling-pointer				\
 		-Wno-zero-length-bounds
 NO_WINFINITE_RECURSION=	-Wno-infinite-recursion