diff --git a/src/low-level/imap/clientid.c b/src/low-level/imap/clientid.c
index 1c34637..f28b616 100644
--- a/src/low-level/imap/clientid.c
+++ b/src/low-level/imap/clientid.c
@@ -33,6 +33,8 @@
 #	include <config.h>
 #endif
 
+#include <stddef.h>
+
 #include "mailimap_sender.h"
 #include "clientid_sender.h"
 #include "clientid.h"