diff --git a/src/lib/small/region.c b/src/lib/small/region.c
index dd13e6e108914119b838ca9d9a8266f84e37c637..a47f7c81dfaff25bf3be721602bcae19fd7cfa13 100644
--- a/src/lib/small/region.c
+++ b/src/lib/small/region.c
@@ -27,6 +27,7 @@
  * SUCH DAMAGE.
  */
 #include "lib/small/region.h"
+#include <sys/types.h> /* ssize_t */
 
 void *
 region_alloc_slow(struct region *region, size_t size)