diff --git a/src/zmalloc.h b/src/zmalloc.h
index 559d27ac2b..8fa150147d 100644
--- a/src/zmalloc.h
+++ b/src/zmalloc.h
@@ -31,6 +31,8 @@
 #ifndef __ZMALLOC_H
 #define __ZMALLOC_H
 
+#include <stddef.h>
+
 /* Double expansion needed for stringification of macro values. */
 #define __xstr(s) __str(s)
 #define __str(s) #s