diff --git a/src/campaign/file.c b/src/campaign/file.c
index 7553d121d2..df37c68e6b 100644
--- a/src/campaign/file.c
+++ b/src/campaign/file.c
@@ -4,6 +4,8 @@
 
 #include "zip/zip.h"
 
+#include <stddef.h>
+
 #define CUSTOM_CAMPAIGN_PREFIX_SIZE sizeof(CUSTOM_CAMPAIGN_DIR_NAME "/")
 
 static struct {