diff --git a/drivers/serial/m32r_sio.c b/drivers/serial/m32r_sio.c
index 348ee2c19b58..c2bb11c02bde 100644
--- a/drivers/serial/m32r_sio.c
+++ b/drivers/serial/m32r_sio.c
@@ -421,7 +421,7 @@ static void transmit_chars(struct uart_sio_port *up)
 		up->port.icount.tx++;
 		if (uart_circ_empty(xmit))
 			break;
-		while (!serial_in(up, UART_LSR) & UART_LSR_THRE);
+		while (!(serial_in(up, UART_LSR) & UART_LSR_THRE));
 
 	} while (--count > 0);