diff --git a/cpu/nrf52/periph/pwm.c b/cpu/nrf52/periph/pwm.c
index f00bca774a..0cd46701e8 100644
--- a/cpu/nrf52/periph/pwm.c
+++ b/cpu/nrf52/periph/pwm.c
@@ -92,7 +92,10 @@ uint32_t pwm_init(pwm_t pwm, pwm_mode_t mode, uint32_t freq, uint16_t res)
     for (unsigned i = 0; i < PWM_CHANNELS; i++) {
         /* either left aligned pol or inverted duty cycle */
         pwm_seq[pwm][i] = (POL_MASK & mode) ? POL_MASK : res;
-        dev(pwm)->PSEL.OUT[i] = pwm_config[pwm].pin[i];
+        /* Sign-extend the undefined pin into a value that also sets the
+         * 'Disconnected' field, and is also that register's reset state */
+        uint32_t extended_pin = (int32_t)(int8_t)pwm_config[pwm].pin[i];
+        dev(pwm)->PSEL.OUT[i] = extended_pin;
         DEBUG("set PIN[%i] to %i with 0x%x\n",
               (int)i, (int)pwm_config[pwm].pin[i], pwm_seq[pwm][i]);
     }
diff --git a/cpu/nrf5x_common/include/periph_cpu_common.h b/cpu/nrf5x_common/include/periph_cpu_common.h
index a467eed50d..d305624d65 100644
--- a/cpu/nrf5x_common/include/periph_cpu_common.h
+++ b/cpu/nrf5x_common/include/periph_cpu_common.h
@@ -55,6 +55,8 @@ extern "C" {
 /**
  * @brief   Override GPIO_UNDEF value
  */
+/* The precise value matters where GPIO_UNDEF is set in registers like
+ * PWM.PSEL.OUT where it is used in sign-extended form to get a UINT32_MAX */
 #define GPIO_UNDEF          (UINT8_MAX)
 
 /**