diff --git a/src/modbus-rtu.c b/src/modbus-rtu.c
index 524c5df..7aa3852 100644
--- a/src/modbus-rtu.c
+++ b/src/modbus-rtu.c
@@ -280,7 +280,7 @@ static ssize_t _modbus_rtu_send(modbus_t *ctx, const uint8_t *req, int req_lengt
 #if defined(_WIN32)
     modbus_rtu_t *ctx_rtu = ctx->backend_data;
     DWORD n_bytes = 0;
-    return (WriteFile(ctx_rtu->w_ser.fd, req, req_length, &n_bytes, NULL)) ? n_bytes : -1;
+    return (WriteFile(ctx_rtu->w_ser.fd, req, req_length, &n_bytes, NULL)) ? (ssize_t)n_bytes : -1;
 #else
 #if HAVE_DECL_TIOCM_RTS
     modbus_rtu_t *ctx_rtu = ctx->backend_data;