diff --git a/symposion/schedule/forms.py b/symposion/schedule/forms.py
index 6b4bf100b12dc5d197c1d061fb3e62bc3b7d82e7..c94da7164b880c465cbd3ea196603cc0cbedb7a3 100644
--- a/symposion/schedule/forms.py
+++ b/symposion/schedule/forms.py
@@ -6,17 +6,18 @@ from symposion.schedule.models import Presentation
 
 class SlotEditForm(forms.Form):
     
-    presentation = forms.ModelChoiceField(
-        queryset=Presentation.objects.all(),
-        required=True,
-    )
+    presentation = forms.ModelChoiceField(queryset=Presentation.objects.all())
     
     def __init__(self, *args, **kwargs):
-        presentation = kwargs.get("initial", {}).get("presentation")
+        content = kwargs.pop("content", None)
+        if content:
+            kwargs.setdefault("initial", {})["presentation"] = content
         super(SlotEditForm, self).__init__(*args, **kwargs)
         queryset = self.fields["presentation"].queryset
-        if presentation:
-            queryset = queryset.filter(Q(slot=None) | Q(pk=presentation.pk))
+        if content:
+            queryset = queryset.filter(Q(slot=None) | Q(pk=content.pk))
+            self.fields["presentation"].required = False
         else:
             queryset = queryset.filter(slot=None)
+            self.fields["presentation"].required = True
         self.fields["presentation"].queryset = queryset
diff --git a/symposion/schedule/models.py b/symposion/schedule/models.py
index 9e9e9159210a1cda952d95eea9f2c7359b19de73..9aac2b30312fda59fa8fd21a47cf6a1cf2a11148 100644
--- a/symposion/schedule/models.py
+++ b/symposion/schedule/models.py
@@ -52,11 +52,17 @@ class Slot(models.Model):
         Assign the given content to this slot and if a previous slot content
         was given we need to unlink it to avoid integrity errors.
         """
+        self.unassign()
+        content.slot = self
+        content.save()
+    
+    def unassign(self):
+        """
+        Unassign the associated content with this slot.
+        """
         if self.content and self.content.slot_id:
             self.content.slot = None
             self.content.save()
-        content.slot = self
-        content.save()
     
     @property
     def content(self):
diff --git a/symposion/schedule/views.py b/symposion/schedule/views.py
index 3c4e0b42bacd89df482ec8f57533ede91d5e89e0..b2e429dcf8749b82606f38364e0e969807b4f945 100644
--- a/symposion/schedule/views.py
+++ b/symposion/schedule/views.py
@@ -67,19 +67,23 @@ def schedule_slot_edit(request, slot_pk):
     
     slot = get_object_or_404(Slot, pk=slot_pk)
     
+    # slot content
+    try:
+        content = slot.content
+    except ObjectDoesNotExist:
+        content = None
+    
     if request.method == "POST":
-        form = SlotEditForm(request.POST)
+        form = SlotEditForm(request.POST, content=content)
         if form.is_valid():
             presentation = form.cleaned_data["presentation"]
-            slot.assign(presentation)
+            if presentation is None:
+                slot.unassign()
+            else:
+                slot.assign(presentation)
         return redirect("schedule_edit_singleton")
     else:
-        initial = {}
-        try:
-            initial["presentation"] = slot.content
-        except ObjectDoesNotExist:
-            pass
-        form = SlotEditForm(initial=initial)
+        form = SlotEditForm(content=content)
         ctx = {
             "form": form,
             "slot": slot,