diff --git a/purchase.py b/purchase.py
index 88225d0..807107f 100644
--- a/purchase.py
+++ b/purchase.py
@@ -162,6 +162,10 @@ class Purchase(metaclass=PoolMeta):
             return
         invoice.reference = self.reference
         invoice.description = self.description
+        if self.invoice_method == 'shipment' and self.moves:
+            for move in self.moves:
+                invoice.reference = move.shipment.reference
+                break
         invoice.save()
         return invoice