diff --git a/os/strcasestr.c b/os/strcasestr.c
index 3189cf21e..4e483a940 100644
--- a/os/strcasestr.c
+++ b/os/strcasestr.c
@@ -37,6 +37,8 @@
 #include <ctype.h>
 #include <string.h>
 
+#include "os.h"
+
 /*
  * Find the first occurrence of find in s, ignore case.
  */