diff --git a/sys/include/net/telnet.h b/sys/include/net/telnet.h
index 544a35872a..6823206fed 100644
--- a/sys/include/net/telnet.h
+++ b/sys/include/net/telnet.h
@@ -72,7 +72,7 @@ int telnet_server_start(void);
  *
  * @return  0 on success, error otherwise
  */
-int telnet_server_write(const void* buffer, size_t len);
+ssize_t telnet_server_write(const void* buffer, size_t len);
 
 /**
  * @brief   Read data from the telnet client, will block until data is available.
diff --git a/sys/net/application_layer/telnet/telnet_server.c b/sys/net/application_layer/telnet/telnet_server.c
index aaaac7dffc..4f0c32e566 100644
--- a/sys/net/application_layer/telnet/telnet_server.c
+++ b/sys/net/application_layer/telnet/telnet_server.c
@@ -293,7 +293,7 @@ disco:
     return NULL;
 }
 
-int telnet_server_write(const void* buffer, size_t len)
+ssize_t telnet_server_write(const void* buffer, size_t len)
 {
     if (connected) {
         int res = _write_buffer(buffer, len);