diff --git a/src/clj/auto_ap/datomic/migrate.clj b/src/clj/auto_ap/datomic/migrate.clj
index bba02b8c..ec46d7c4 100644
--- a/src/clj/auto_ap/datomic/migrate.clj
+++ b/src/clj/auto_ap/datomic/migrate.clj
@@ -55,12 +55,18 @@
     :db/doc "Data function that increments value of attribute a by amount."
     :db/fn (d/function '{:lang "clojure"
                         :params [db e amount]
-                        :code (let [current-outstanding-balance (-> (d/entity db e) :invoice/outstanding-balance)
-                                new-outstanding-balance (- current-outstanding-balance amount)]
-                            [[:db/add e :invoice/outstanding-balance new-outstanding-balance]
-                             [:db/add e :invoice/status (if (< -0.001 new-outstanding-balance 0.001)
-                                                          :invoice-status/paid
-                                                          :invoice-status/unpaid)]])})}]] )
+                         :code (let [invoice                     (-> (d/entity db e))
+                                     current-outstanding-balance (-> invoice :invoice/outstanding-balance)
+                                     new-outstanding-balance     (- current-outstanding-balance amount)]
+                                 (when (= (:invoice/status invoice)
+                                          :invoice-status/paid)
+                                   (throw (ex-info "This invoice has already been paid" {:validation-message "This invoice has already been paid."
+                                                                                         :status             (:invoice/status invoice)
+                                                                                         :id (:db/id invoice)})))
+                                 [[:db/add e :invoice/outstanding-balance new-outstanding-balance]
+                                  [:db/add e :invoice/status (if (< -0.001 new-outstanding-balance 0.001)
+                                                               :invoice-status/paid
+                                                               :invoice-status/unpaid)]])})}]] )
 
 (def add-client-identifier
   [[{:db/ident :invoice/client-identifier
@@ -165,7 +171,7 @@
         norms-map (merge {:auto-ap/base-schema {:txes auto-ap.datomic/base-schema}
                           :auto-ap/functions {:txes-fn 'auto-ap.datomic.migrate/functions
                                               :requires [:auto-ap/base-schema]}
-                          :auto-ap/fx-pay-function-2 {:txes-fn 'auto-ap.datomic.migrate/fix-pay-function
+                          :auto-ap/fx-pay-function-10 {:txes-fn 'auto-ap.datomic.migrate/fix-pay-function
                                                       :requires [:auto-ap/functions]}
                           :auto-ap/migrate-vendors {:txes-fn 'auto-ap.datomic/migrate-vendors
                                                     :requires [:auto-ap/base-schema]}