diff --git a/ports/atmel-samd/common-hal/microcontroller/Processor.c b/ports/atmel-samd/common-hal/microcontroller/Processor.c
index 1c57808f7d..5c0d72c96a 100644
--- a/ports/atmel-samd/common-hal/microcontroller/Processor.c
+++ b/ports/atmel-samd/common-hal/microcontroller/Processor.c
@@ -243,7 +243,7 @@ void common_hal_mcu_processor_get_uid(uint8_t raw_id[]) {
 
     for (int i=0; i<4; i++) {
         for (int k=0; k<4; k++) {
-            raw_id[4 * i + k] = (*(id_addresses[i]) >> k * 8) & 0xf;
+            raw_id[4 * i + k] = (*(id_addresses[i]) >> k * 8) & 0xff;
         }
     }
 }
diff --git a/ports/nrf/common-hal/microcontroller/Processor.c b/ports/nrf/common-hal/microcontroller/Processor.c
index cf808cdf4c..0cceb6a665 100644
--- a/ports/nrf/common-hal/microcontroller/Processor.c
+++ b/ports/nrf/common-hal/microcontroller/Processor.c
@@ -76,7 +76,7 @@ void common_hal_mcu_processor_get_uid(uint8_t raw_id[]) {
 
     for (int i=0; i<2; i++) {
         for (int k=0; k<4; k++) {
-            raw_id[4 * i + k] = (*(id_addresses[i]) >> k * 8) & 0xf;
+            raw_id[4 * i + k] = (*(id_addresses[i]) >> k * 8) & 0xff;
         }
     }
 }