diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -276,14 +276,10 @@
 
 CSTD?=		gnu17
 
-.if ${CSTD} == "k&r"
-CFLAGS+=        -traditional
-.elif ${CSTD} == "c89" || ${CSTD} == "c90"
-CFLAGS+=        -std=iso9899:1990
-.elif ${CSTD} == "c94" || ${CSTD} == "c95"
-CFLAGS+=        -std=iso9899:199409
-.elif ${CSTD} == "c99"
-CFLAGS+=        -std=iso9899:1999
+# c99/gnu99 is the minimum C standard version supported for kernel build
+.if ${CSTD} == "k&r" || ${CSTD} == "c89" || ${CSTD} == "c90" || \
+    ${CSTD} == "c94" || ${CSTD} == "c95"
+.error "Only c99/gnu99 or later is supported"
 .else # CSTD
 CFLAGS+=        -std=${CSTD}
 .endif # CSTD