diff --git a/purchase.py b/purchase.py
index 4e7ec7ef71..6643431266 100644
--- a/purchase.py
+++ b/purchase.py
@@ -101,7 +101,7 @@ class PurchaseLine(ModelSQL, ModelView):
             for line in lines:
                 for root_id in root_ids:
                     id2record[line.id]['analytic_account_' \
-                            + str(root_id)] = False
+                            + str(root_id)] = None
                 if line.type != 'line':
                     continue
                 if not line.analytic_accounts: