diff --git a/core/net/ip64/ip64-eth-interface.c b/core/net/ip64/ip64-eth-interface.c
index 42f6234cc..026467137 100644
--- a/core/net/ip64/ip64-eth-interface.c
+++ b/core/net/ip64/ip64-eth-interface.c
@@ -84,7 +84,7 @@ init(void)
   printf("ip64-eth-interface: init\n");
 }
 /*---------------------------------------------------------------------------*/
-static void
+static int
 output(void)
 {
   int len, ret;
@@ -114,7 +114,7 @@ output(void)
       printf("Create request\n");
       len = ip64_arp_create_arp_request(ip64_packet_buffer,
 					&ip64_packet_buffer[sizeof(struct ip64_eth_hdr)]);
-      IP64_ETH_DRIVER.output(ip64_packet_buffer, len);
+      return IP64_ETH_DRIVER.output(ip64_packet_buffer, len);
     }
   }
 }
diff --git a/core/net/ip64/ip64-slip-interface.c b/core/net/ip64/ip64-slip-interface.c
index 8d46d78a8..f62309cab 100644
--- a/core/net/ip64/ip64-slip-interface.c
+++ b/core/net/ip64/ip64-slip-interface.c
@@ -115,7 +115,7 @@ init(void)
   slip_set_input_callback(input_callback);
 }
 /*---------------------------------------------------------------------------*/
-static void
+static int
 output(void)
 {
   int len;
@@ -138,8 +138,10 @@ output(void)
       memcpy(&uip_buf[UIP_LLH_LEN], ip64_packet_buffer, len);
       uip_len = len;
       slip_send();
+      return len;
     }
   }
+  return 0;
 }
 /*---------------------------------------------------------------------------*/
 const struct uip_fallback_interface ip64_slip_interface = {