diff --git a/src/api/err.c b/src/api/err.c
index e1ef344f..cce723be 100644
--- a/src/api/err.c
+++ b/src/api/err.c
@@ -38,6 +38,7 @@
 
 #include "lwip/err.h"
 #include "lwip/def.h"
+#include "lwip/sys.h"
 
 #include "lwip/errno.h"