diff --git a/src/core/def.c b/src/core/def.c
index 1f8a4b96..c0cfba3b 100644
--- a/src/core/def.c
+++ b/src/core/def.c
@@ -49,6 +49,8 @@
 #include "lwip/opt.h"
 #include "lwip/def.h"
 
+#include <string.h>
+
 #if (LWIP_PLATFORM_BYTESWAP == 0) && (BYTE_ORDER == LITTLE_ENDIAN)
 
 /**