Index: head/sys/kern/kern_sysctl.c
===================================================================
--- head/sys/kern/kern_sysctl.c
+++ head/sys/kern/kern_sysctl.c
@@ -1784,7 +1784,7 @@
 		ktrsysctl(name, namelen);
 #endif
 
-	if (req.oldlen > PAGE_SIZE) {
+	if (req.oldptr && req.oldlen > PAGE_SIZE) {
 		memlocked = 1;
 		sx_xlock(&sysctlmemlock);
 	} else