diff --git a/lib/libmemstat/memstat_uma.c b/lib/libmemstat/memstat_uma.c
index 7c292e7ba46..cf30f5f312e 100644
--- a/lib/libmemstat/memstat_uma.c
+++ b/lib/libmemstat/memstat_uma.c
@@ -31,6 +31,7 @@
 #include <sys/param.h>
 #include <sys/counter.h>
 #include <sys/cpuset.h>
+#include <sys/stddef.h>
 #include <sys/sysctl.h>
 
 #include <vm/uma.h>