diff --git a/libraries/AP_HAL_ChibiOS/RCOutput.cpp b/libraries/AP_HAL_ChibiOS/RCOutput.cpp
index f4ba0ba7525..72e82e3a0c5 100644
--- a/libraries/AP_HAL_ChibiOS/RCOutput.cpp
+++ b/libraries/AP_HAL_ChibiOS/RCOutput.cpp
@@ -2186,7 +2186,7 @@ void RCOutput::serial_bit_irq(void)
 void RCOutput::serial_byte_timeout(virtual_timer_t* vt, void *ctx)
 {
     chSysLockFromISR();
-    uint16_t byteval = irq.bitmask | 0x200;
+    uint16_t byteval = irq.bitmask | (((1U<<BYTE_BITS)-1) & ~((1U<<irq.nbits)-1));
     // we can accept a byte with a timeout if the last bit was 1
     // and the start bit is set correctly
     if (irq.last_bit == 0) {