diff --git a/src/string_t.c b/src/string_t.c
index b94f1203..01f48659 100644
--- a/src/string_t.c
+++ b/src/string_t.c
@@ -34,6 +34,7 @@
 #define _GNU_SOURCE		/* to get iswblank */
 
 #include <assert.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <wchar.h>