diff --git a/library/psa_crypto_slot_management.c b/library/psa_crypto_slot_management.c
index 358c7a2f2f..f1a651f0d9 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -663,7 +663,7 @@ psa_status_t psa_reserve_free_key_slot(psa_key_id_t *volatile_key_id,
             /* Refresh slot_idx, for when the slot is not the original
              * selected_slot but rather unused_persistent_key_slot.  */
             slot_idx = selected_slot - global_data.key_slots;
-            *volatile_key_id = PSA_KEY_ID_VOLATILE_MIN + slot_idx;
+            *volatile_key_id = PSA_KEY_ID_VOLATILE_MIN + (psa_key_id_t) slot_idx;
         }
 #endif
         *p_slot = selected_slot;