diff --git a/account_invoice_merge/models/account_invoice.py b/account_invoice_merge/models/account_invoice.py
index 4e27bf7ffb6..df40c58a357 100644
--- a/account_invoice_merge/models/account_invoice.py
+++ b/account_invoice_merge/models/account_invoice.py
@@ -53,6 +53,11 @@ def _get_first_invoice_fields(self, invoice):
             'partner_bank_id': invoice.partner_bank_id.id,
         }
 
+    @api.multi
+    def _get_draft_invoices(self):
+        """Overridable function to return draft invoices to merge"""
+        return self.filtered(lambda x: x.state == 'draft')
+
     @api.multi
     def do_merge(self, keep_references=True, date_invoice=False,
                  remove_empty_invoice_lines=True):
@@ -96,13 +101,10 @@ def make_key(br, fields):
 
         # compute what the new invoices should contain
         new_invoices = {}
-        draft_invoices = [invoice
-                          for invoice in self
-                          if invoice.state == 'draft']
         seen_origins = {}
         seen_client_refs = {}
 
-        for account_invoice in draft_invoices:
+        for account_invoice in self._get_draft_invoices():
             invoice_key = make_key(
                 account_invoice, self._get_invoice_key_cols())
             new_invoice = new_invoices.setdefault(invoice_key, ({}, []))
diff --git a/account_invoice_merge/wizard/invoice_merge.py b/account_invoice_merge/wizard/invoice_merge.py
index 3a86e6b0076..c7ecad3783b 100644
--- a/account_invoice_merge/wizard/invoice_merge.py
+++ b/account_invoice_merge/wizard/invoice_merge.py
@@ -19,6 +19,20 @@ class InvoiceMerge(models.TransientModel):
                                      default=True)
     date_invoice = fields.Date('Invoice Date')
 
+    @api.model
+    def _get_not_mergeable_invoices_message(self, invoices):
+        """Overridable function to custom error message"""
+        key_fields = invoices._get_invoice_key_cols()
+        error_msg = {}
+        if len(invoices) != len(invoices._get_draft_invoices()):
+            error_msg['state'] = (
+                _('Megeable State (ex : %s)') %
+                (invoices and invoices[0].state or _('Draf')))
+        for field in key_fields:
+            if len(set(invoices.mapped(field))) > 1:
+                error_msg[field] = invoices._fields[field].string
+        return error_msg
+
     @api.model
     def _dirty_check(self):
         if self.env.context.get('active_model', '') == 'account.invoice':
@@ -29,29 +43,11 @@ def _dirty_check(self):
                       'view.'))
 
             invs = self.env['account.invoice'].browse(ids)
-            for d in invs:
-                if d['state'] != 'draft':
-                    raise UserError(
-                        _('At least one of the selected invoices is %s!') %
-                        d['state'])
-                if d['account_id'] != invs[0]['account_id']:
-                    raise UserError(
-                        _('Not all invoices use the same account!'))
-                if d['company_id'] != invs[0]['company_id']:
-                    raise UserError(
-                        _('Not all invoices are at the same company!'))
-                if d['partner_id'] != invs[0]['partner_id']:
-                    raise UserError(
-                        _('Not all invoices are for the same partner!'))
-                if d['type'] != invs[0]['type']:
-                    raise UserError(
-                        _('Not all invoices are of the same type!'))
-                if d['currency_id'] != invs[0]['currency_id']:
-                    raise UserError(
-                        _('Not all invoices are at the same currency!'))
-                if d['journal_id'] != invs[0]['journal_id']:
-                    raise UserError(
-                        _('Not all invoices are at the same journal!'))
+            error_msg = self._get_not_mergeable_invoices_message(invs)
+            if error_msg:
+                all_msg = _("All invoices must have the same: \n")
+                all_msg += '\n'.join([value for value in error_msg.values()])
+                raise UserError(all_msg)
         return {}
 
     @api.model