diff --git a/freemodbus/tcp_slave/port/port_tcp_slave.c b/freemodbus/tcp_slave/port/port_tcp_slave.c
index 616d0f2..891da72 100644
--- a/freemodbus/tcp_slave/port/port_tcp_slave.c
+++ b/freemodbus/tcp_slave/port/port_tcp_slave.c
@@ -60,7 +60,7 @@
 #if MB_TCP_ENABLED
 
 /* ----------------------- Defines  -----------------------------------------*/
-#define MB_TCP_DISCONNECT_TIMEOUT       ( CONFIG_FMB_TCP_CONNECTION_TOUT_SEC * 1000000 ) // disconnect timeout in uS
+#define MB_TCP_DISCONNECT_TIMEOUT       ( CONFIG_FMB_TCP_CONNECTION_TOUT_SEC * 1000000UL ) // disconnect timeout in uS
 #define MB_TCP_RESP_TIMEOUT_MS          ( MB_MASTER_TIMEOUT_MS_RESPOND - 1 ) // slave response time limit
 #define MB_TCP_NET_LISTEN_BACKLOG       ( SOMAXCONN )