diff --git a/library/psa_crypto_slot_management.c b/library/psa_crypto_slot_management.c
index dad23c4..dbe3bba 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -182,6 +182,29 @@
     psa_free_persistent_key_data( key_data, key_data_length );
     return( status );
 }
+
+/** Check whether a key identifier is acceptable.
+ *
+ * For backward compatibility, key identifiers that were valid in a
+ * past released version must remain valid, unless a migration path
+ * is provided.
+ *
+ * \param key_id        The key identifier to check.
+ *
+ * \return              1 if \p key_id is acceptable, otherwise 0.
+ */
+static int psa_is_key_id_valid( psa_key_id_t key_id )
+{
+    /* Reject id=0 because by general library conventions, 0 is an invalid
+     * value wherever possible. */
+    if( key_id == 0 )
+        return( 0 );
+    /* Reject high values because the file names are reserved for the
+     * library's internal use. */
+    if( key_id >= PSA_MAX_PERSISTENT_KEY_IDENTIFIER )
+        return( 0 );
+    return( 1 );
+}
 #endif /* defined(MBEDTLS_PSA_CRYPTO_STORAGE_C) */
 
 /** Declare a slot as persistent and load it from storage.
@@ -209,13 +232,7 @@
     psa_key_slot_t *slot;
     psa_status_t status;
 
-    /* Reject id=0 because by general library conventions, 0 is an invalid
-     * value wherever possible. */
-    if( id == 0 )
-        return( PSA_ERROR_INVALID_ARGUMENT );
-    /* Reject high values because the file names are reserved for the
-     * library's internal use. */
-    if( id >= PSA_MAX_PERSISTENT_KEY_IDENTIFIER )
+    if( ! psa_is_key_id_valid( id ) )
         return( PSA_ERROR_INVALID_ARGUMENT );
 
     status = psa_get_key_slot( handle, &slot );