diff --git a/ports/atmel-samd/common-hal/busio/UART.c b/ports/atmel-samd/common-hal/busio/UART.c
index 4cb7db2033..5376775206 100644
--- a/ports/atmel-samd/common-hal/busio/UART.c
+++ b/ports/atmel-samd/common-hal/busio/UART.c
@@ -56,7 +56,7 @@ void common_hal_busio_uart_construct(busio_uart_obj_t *self,
         uint8_t bits, uart_parity_t parity, uint8_t stop, uint32_t timeout,
         uint8_t receiver_buffer_size) {
     Sercom* sercom = NULL;
-    uint8_t sercom_index;
+    uint8_t sercom_index = 255; // Unset index
     uint32_t rx_pinmux = 0;
     uint8_t rx_pad = 255; // Unset pad
     uint32_t tx_pinmux = 0;