diff --git a/src/apps/tftp/tftp.c b/src/apps/tftp/tftp.c
index ddfdbfc0..aab1d9ce 100644
--- a/src/apps/tftp/tftp.c
+++ b/src/apps/tftp/tftp.c
@@ -157,7 +157,7 @@ send_request(const ip_addr_t *addr, u16_t port, u16_t opcode, const char* fname,
 static err_t
 send_error(const ip_addr_t *addr, u16_t port, enum tftp_error code, const char *str)
 {
-  int str_length = strlen(str);
+  size_t str_length = strlen(str);
   struct pbuf *p;
   u16_t *payload;
   err_t ret;