Allow orders to be created through Rules with a NULL order_number

Hi I have a set of rules set up to create a subscription service for a groceries shop. The real-life scenario is:
1- customer adds the subscription product (a basket with groceries) into his shopping cart
2- customer completes checkout
3- upon completing checkout a Rule loops over the products and checks whether a subscription product is bought
4- if a subscription product has been bought schedule a rule for next monday which creates a new order with the subscription product and completes checkout in name of this client
5- back to step 3 (ad infinitum)

The problem is that I get an error after cron (when the scheduled rule has run):

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '1000000' for key 'order_number': INSERT INTO {commerce_order} (order_number, revision_id, type, uid, mail, status, created, changed, hostname, data) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9); Array ( [:db_insert_placeholder_0] => 1000000 [:db_insert_placeholder_1] => [:db_insert_placeholder_2] => commerce_order [:db_insert_placeholder_3] => 1 [:db_insert_placeholder_4] => [:db_insert_placeholder_5] => cart [:db_insert_placeholder_6] => 1354897305 [:db_insert_placeholder_7] => 1354897305 [:db_insert_placeholder_8] => 188.205.30.132 [:db_insert_placeholder_9] => a:0:{} ) in drupal_write_record() (line 7106 of /srv/brammer/www/commerce.ruudmaaz.nl/includes/common.inc).

The Rule Component that creates the order is:

{ "rules_voeg_abonnement_toe_aan_cart" : {
    "LABEL" : "Voeg Abonnement toe aan cart",
    "PLUGIN" : "rule",
    "REQUIRES" : [ "rules", "commerce_cart", "commerce_checkout" ],
    "USES VARIABLES" : {
      "user" : { "label" : "User", "type" : "user" },
      "product" : { "label" : "Product", "type" : "commerce_product" }
    },
    "IF" : [
      { "entity_has_field" : { "entity" : [ "user" ], "field" : "field_abonnementen" } }
    ],
    "DO" : [
      { "entity_create" : {
          "USING" : {
            "type" : "commerce_order",
            "param_order_number" : "1000000",
            "param_status" : "cart",
            "param_type" : "commerce_order",
            "param_owner" : [ "user" ],
            "param_commerce_order_total" : [ "product:commerce-price" ]
          },
          "PROVIDE" : { "entity_created" : { "order_created" : "Created order" } }
        }
      },
      { "commerce_cart_product_add_by_sku" : {
          "USING" : {
            "user" : [ "user" ],
            "sku" : [ "product:sku" ],
            "quantity" : "1",
            "combine" : 1
          },
          "PROVIDE" : { "product_add_line_item" : { "product_add_line_item" : "Added product line item" } }
        }
      },
      { "commerce_checkout_complete" : { "commerce_order" : [ "product-add-line-item:order" ] } }
    ],
    "PROVIDES VARIABLES" : [ "user", "product" ]
  }
}

What should I enter for Order-number? I tried using a high number 10000 as this comment said it doesn't matter what you enter here.: http://drupal.org/node/1109758#comment-4279684

Cheers,
Bram.