diff --git a/arch/arm/src/nrf52/nrf52_uid.c b/arch/arm/src/nrf52/nrf52_uid.c
index cf53b719d4..845bc1e419 100644
--- a/arch/arm/src/nrf52/nrf52_uid.c
+++ b/arch/arm/src/nrf52/nrf52_uid.c
@@ -33,7 +33,7 @@
  * Public Functions
  ****************************************************************************/
 
-void nrf52_get_uniqueid(uint8_t uniqueid[])
+void nrf52_get_uniqueid(uint8_t uniqueid[8])
 {
   uint32_t uid0 = getreg32(NRF52_FICR_BASE + NRF52_FICR_DEVICEID0_OFFSET);
   uint32_t uid1 = getreg32(NRF52_FICR_BASE + NRF52_FICR_DEVICEID1_OFFSET);
diff --git a/arch/arm/src/nrf52/nrf52_uid.h b/arch/arm/src/nrf52/nrf52_uid.h
index 72dc1e3622..5fe7489b1a 100644
--- a/arch/arm/src/nrf52/nrf52_uid.h
+++ b/arch/arm/src/nrf52/nrf52_uid.h
@@ -31,6 +31,6 @@
  * Public Function Prototypes
  ****************************************************************************/
 
-void nrf52_get_uniqueid(uint8_t uniqueid[]);
+void nrf52_get_uniqueid(uint8_t uniqueid[8]);
 
 #endif /* __ARCH_ARM_SRC_NRF52_NRF52_UID_H */
diff --git a/arch/arm/src/nrf53/nrf53_uid.c b/arch/arm/src/nrf53/nrf53_uid.c
index c471d99251..a62c4c3a18 100644
--- a/arch/arm/src/nrf53/nrf53_uid.c
+++ b/arch/arm/src/nrf53/nrf53_uid.c
@@ -33,7 +33,7 @@
  * Public Functions
  ****************************************************************************/
 
-void nrf53_get_uniqueid(uint8_t uniqueid[])
+void nrf53_get_uniqueid(uint8_t uniqueid[8])
 {
   uint32_t uid0 = getreg32(NRF53_FICR_BASE +
                            NRF53_FICR_INFO_DEVICEID0_OFFSET);
diff --git a/arch/arm/src/nrf53/nrf53_uid.h b/arch/arm/src/nrf53/nrf53_uid.h
index 4d53e8baed..c749bf82f4 100644
--- a/arch/arm/src/nrf53/nrf53_uid.h
+++ b/arch/arm/src/nrf53/nrf53_uid.h
@@ -31,6 +31,6 @@
  * Public Function Prototypes
  ****************************************************************************/
 
-void nrf53_get_uniqueid(uint8_t uniqueid[]);
+void nrf53_get_uniqueid(uint8_t uniqueid[8]);
 
 #endif /* __ARCH_ARM_SRC_NRF53_NRF53_UID_H */