diff --git a/polymorphic/admin/helpers.py b/polymorphic/admin/helpers.py
index 87c56d0..8790a3c 100644
--- a/polymorphic/admin/helpers.py
+++ b/polymorphic/admin/helpers.py
@@ -22,6 +22,10 @@ class PolymorphicInlineAdminForm(InlineAdminForm):
     def polymorphic_ctype_field(self):
         return AdminField(self.form, 'polymorphic_ctype', False)
 
+    @property
+    def is_empty(self):
+        return '__prefix__' in self.form.prefix
+
 
 class PolymorphicInlineAdminFormSet(InlineAdminFormSet):
     """
diff --git a/polymorphic/templates/admin/polymorphic/edit_inline/stacked.html b/polymorphic/templates/admin/polymorphic/edit_inline/stacked.html
index 8dc4d3b..ea929e2 100644
--- a/polymorphic/templates/admin/polymorphic/edit_inline/stacked.html
+++ b/polymorphic/templates/admin/polymorphic/edit_inline/stacked.html
@@ -11,7 +11,7 @@
 {{ inline_admin_formset.formset.non_form_errors }}
 
 {% for inline_admin_form in inline_admin_formset %}
-  <div class="inline-related inline-{{ inline_admin_form.model_admin.opts.model_name }}{% if inline_admin_form.original or inline_admin_form.show_url %} has_original{% endif %}{% if not inline_admin_form.original.pk %} empty-form {% endif %}{% if forloop.last %} last-related{% endif %}"
+  <div class="inline-related inline-{{ inline_admin_form.model_admin.opts.model_name }}{% if inline_admin_form.original or inline_admin_form.show_url %} has_original{% endif %}{% if inline_admin_form.is_empty %} empty-form {% endif %}{% if forloop.last %} last-related{% endif %}"
        id="{% if inline_admin_form.original.pk %}{{ inline_admin_formset.formset.prefix }}-{{ forloop.counter0 }}{% else %}{{ inline_admin_form.model_admin.opts.model_name }}-empty{% endif %}">
 
     <h3><b>{{ inline_admin_form.model_admin.opts.verbose_name|capfirst }}:</b>&nbsp;<span class="inline_label">{% if inline_admin_form.original %}{{ inline_admin_form.original }}{% if inline_admin_form.model_admin.show_change_link and inline_admin_form.model_admin.has_registered_model %} <a href="{% url inline_admin_form.model_admin.opts|admin_urlname:'change' inline_admin_form.original.pk|admin_urlquote %}" class="inlinechangelink">{% trans "Change" %}</a>{% endif %}