diff --git a/helpdesk/forms.py b/helpdesk/forms.py
index bb9072a0..d6779c69 100644
--- a/helpdesk/forms.py
+++ b/helpdesk/forms.py
@@ -515,14 +515,17 @@ class TicketDependencyForm(forms.ModelForm):
 class MultipleTicketSelectForm(forms.Form):
     tickets = forms.ModelMultipleChoiceField(
         label=_('Tickets to merge'),
-        queryset=Ticket.objects.all(),
+        queryset=Ticket.objects.filter(merged_to=None),
         widget=forms.SelectMultiple(attrs={'class': 'form-control'})
     )
 
     def clean_tickets(self):
         tickets = self.cleaned_data.get('tickets')
         if len(tickets) < 2:
-            raise ValidationError(_('Please choose at least 2 tickets'))
+            raise ValidationError(_('Please choose at least 2 tickets.'))
         if len(tickets) > 4:
             raise ValidationError(_('Impossible to merge more than 4 tickets...'))
+        queues = tickets.order_by('queue').distinct().values_list('queue', flat=True)
+        if len(queues) != 1:
+            raise ValidationError(_('All selected tickets must share the same queue in order to be merged.'))
         return tickets