diff --git a/addons/website/models/website.py b/addons/website/models/website.py
index 9c864461a83..453596ad76b 100644
--- a/addons/website/models/website.py
+++ b/addons/website/models/website.py
@@ -122,8 +122,7 @@ def slugify(s, max_length=None):
 def slug(value):
     if isinstance(value, models.BaseModel):
         if isinstance(value.id, models.NewId):
-            # it's not possible to slug a record if it's not saved
-            return None
+            raise ValueError("Cannot slug non-existent record %s" % value)
         # [(id, name)] = value.name_get()
         identifier, name = value.id, value.display_name
     else:
diff --git a/addons/website_event/models/event.py b/addons/website_event/models/event.py
index 1fdc7bb8ff6..6c1e1ddf809 100644
--- a/addons/website_event/models/event.py
+++ b/addons/website_event/models/event.py
@@ -43,7 +43,8 @@ class Event(models.Model):
     def _compute_website_url(self):
         super(Event, self)._compute_website_url()
         for event in self:
-            event.website_url = '/event/%s' % slug(event)
+            if event.id:  # avoid to perform a slug on a not yet saved record in case of an onchange.
+                event.website_url = '/event/%s' % slug(event)
 
     @api.multi
     def _get_new_menu_pages(self):
diff --git a/addons/website_slides/models/slides.py b/addons/website_slides/models/slides.py
index bc64375e36c..b2e6fb0ae7f 100644
--- a/addons/website_slides/models/slides.py
+++ b/addons/website_slides/models/slides.py
@@ -148,9 +148,8 @@ class Channel(models.Model):
         super(Channel, self)._compute_website_url()
         base_url = self.env['ir.config_parameter'].get_param('web.base.url')
         for channel in self:
-            channel_slug = slug(channel)
-            if channel_slug:
-                channel.website_url = '%s/slides/%s' % (base_url, channel_slug)
+            if channel.id:  # avoid to perform a slug on a not yet saved record in case of an onchange.
+                channel.website_url = '%s/slides/%s' % (base_url, slug(channel))
 
     @api.onchange('visibility')
     def change_visibility(self):
@@ -354,13 +353,12 @@ class Slide(models.Model):
         super(Slide, self)._compute_website_url()
         base_url = self.env['ir.config_parameter'].get_param('web.base.url')
         for slide in self:
-            slide_slug = slug(slide)
-            if slide_slug:
-                #link_tracker is not in dependencies, so use it to shorten url only if installed.
+            if slide.id:  # avoid to perform a slug on a not yet saved record in case of an onchange.
+                # link_tracker is not in dependencies, so use it to shorten url only if installed.
                 if self.env.registry.get('link.tracker'):
-                    url = self.env['link.tracker'].sudo().create({'url': '%s/slides/slide/%s' % (base_url, slide_slug)}).short_url
+                    url = self.env['link.tracker'].sudo().create({'url': '%s/slides/slide/%s' % (base_url, slug(slide))}).short_url
                 else:
-                    url = '%s/slides/slide/%s' % (base_url, slide_slug)
+                    url = '%s/slides/slide/%s' % (base_url, slug(slide))
                 slide.website_url = url
 
     @api.model