diff --git a/features/FEATURE_BLE/targets/TARGET_NORDIC/TARGET_MCU_NRF51822/sdk/source/ble/peer_manager/pm_buffer.c b/features/FEATURE_BLE/targets/TARGET_NORDIC/TARGET_MCU_NRF51822/sdk/source/ble/peer_manager/pm_buffer.c
index 87379e3ac0..4299793732 100644
--- a/features/FEATURE_BLE/targets/TARGET_NORDIC/TARGET_MCU_NRF51822/sdk/source/ble/peer_manager/pm_buffer.c
+++ b/features/FEATURE_BLE/targets/TARGET_NORDIC/TARGET_MCU_NRF51822/sdk/source/ble/peer_manager/pm_buffer.c
@@ -93,7 +93,7 @@ uint8_t pm_buffer_block_acquire(pm_buffer_t * p_buffer, uint32_t n_blocks)
             {
                 first_locked_mutex = i;
             }
-            if ((i - first_locked_mutex + 1) == n_blocks)
+            if ((i - first_locked_mutex + 1) >= 0 && ((uint32_t)(i - first_locked_mutex + 1)) == n_blocks)
             {
                 return first_locked_mutex;
             }
diff --git a/hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/sdk/ble/peer_manager/pm_buffer.c b/hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/sdk/ble/peer_manager/pm_buffer.c
index 20ea66b521..f185b5358e 100644
--- a/hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/sdk/ble/peer_manager/pm_buffer.c
+++ b/hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/sdk/ble/peer_manager/pm_buffer.c
@@ -100,7 +100,7 @@ uint8_t pm_buffer_block_acquire(pm_buffer_t * p_buffer, uint32_t n_blocks)
             {
                 first_locked_mutex = i;
             }
-            if ((i - first_locked_mutex + 1) == n_blocks)
+            if ((i - first_locked_mutex + 1) >= 0 && ((uint32_t) (i - first_locked_mutex + 1)) == n_blocks)
             {
                 return first_locked_mutex;
             }