diff --git a/usr.sbin/acpi/acpidb/Makefile b/usr.sbin/acpi/acpidb/Makefile
index 493f2b897d06..513e03e8b5d5 100644
--- a/usr.sbin/acpi/acpidb/Makefile
+++ b/usr.sbin/acpi/acpidb/Makefile
@@ -81,6 +81,12 @@ LIBADD=	pthread
 
 .include <bsd.prog.mk>
 
+# clang 6.0.0 and higher warn about the ACPI_ROOT_OBJECT and
+# ACPI_TO_POINTER macros from sys/contrib/dev/acpica/include/actypes.h,
+# that they use arithmetic on a null pointer treated as a cast from
+# integer to pointer, which is a GNU extension.
+#
+# Turn off the warning, because this is in contributed code.
 .if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 60000
 CWARNFLAGS+=	-Wno-null-pointer-arithmetic
 .endif