diff --git a/ports/nrf/modules/machine/adc.c b/ports/nrf/modules/machine/adc.c
index bbb5221540..d863aebb35 100644
--- a/ports/nrf/modules/machine/adc.c
+++ b/ports/nrf/modules/machine/adc.c
@@ -97,7 +97,7 @@ STATIC int adc_find(mp_obj_t id) {
     int adc_idx = adc_id;
 
     if (adc_idx >= 0 && adc_idx <= MP_ARRAY_SIZE(machine_adc_obj)
-        && machine_adc_obj[adc_idx].id != -1) {
+        && machine_adc_obj[adc_idx].id != (uint8_t)-1) {
         return adc_idx;
     }
     nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError,