diff --git a/invoice.py b/invoice.py
index ac1a28f7bc..6ced10de3e 100644
--- a/invoice.py
+++ b/invoice.py
@@ -28,7 +28,7 @@ class InvoiceLine:
                     ])
             if vals['invoice']:
                 Purchase.write(purchases, {
-                    'invoices': [('add', vals['invoice'])],
+                    'invoices': [('add', [vals['invoice']])],
                     })
             else:
                 for purchase in purchases: