diff --git a/__init__.py b/__init__.py
index ab3c91b..be336a1 100644
--- a/__init__.py
+++ b/__init__.py
@@ -17,8 +17,6 @@ def register():
         party.Party,
         company.Company,
         invoice.Invoice,
-        invoice.Sale,
-        invoice.Purchase,
         load_pkcs12.LoadPKCS12Start,
         aeat.SIIReport,
         aeat.SIIReportLine,
@@ -26,6 +24,14 @@ def register():
         aeat_mapping.IssuedTrytonInvoiceMapper,
         aeat_mapping.RecievedTrytonInvoiceMapper,
         module='aeat_sii', type_='model')
+    Pool.register(
+        invoice.Sale,
+        depends=['sale'],
+        module='aeat_sii', type_='model')
+    Pool.register(
+        invoice.Purchase,
+        depends=['purchase'],
+        module='aeat_sii', type_='model')
     Pool.register(
         load_pkcs12.LoadPKCS12,
         module='aeat_sii', type_='wizard')