diff --git a/addons/sale_timesheet/models/account.py b/addons/sale_timesheet/models/account.py
index b933b65363f..c14767aacbf 100644
--- a/addons/sale_timesheet/models/account.py
+++ b/addons/sale_timesheet/models/account.py
@@ -121,6 +121,10 @@ class AccountAnalyticLine(models.Model):
         domain = super(AccountAnalyticLine, self)._timesheet_get_portal_domain()
         return expression.AND([domain, [('timesheet_invoice_type', 'in', ['billable_time', 'non_billable'])]])
 
+    def _get_timesheets_to_merge(self):
+        res = super(AccountAnalyticLine, self)._get_timesheets_to_merge()
+        return res.filtered(lambda l: not l.timesheet_invoice_id or l.timesheet_invoice_id.state != 'posted')
+
     def unlink(self):
         if any(line.timesheet_invoice_id and line.timesheet_invoice_id.state == 'posted' for line in self):
             raise UserError(_('You cannot remove a timesheet that has already been invoiced.'))