diff --git a/docs/changelog.rst b/docs/changelog.rst
index 9c932e5..62c6227 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -8,6 +8,7 @@ Version 0.5.2 (unreleased)
 * Fix unwanted ``___`` handling in the ORM when a field name starts with an underscore;
   this detects you meant ``relatedfield__ _underscorefield`` instead of ``ClassName___field``.
 * Fix missing permission check in the "add type" view. This was caught however in the next step.
+* Fix admin validation errors related to additional non-model form fields.
 
 
 Version 0.5.1 (2013-07-05)
diff --git a/polymorphic/admin.py b/polymorphic/admin.py
index 9bf80de..019415b 100644
--- a/polymorphic/admin.py
+++ b/polymorphic/admin.py
@@ -479,6 +479,11 @@ class PolymorphicChildModelAdmin(admin.ModelAdmin):
             return self.base_fieldsets
 
 
+    def get_form(self, request, obj=None, **kwargs):
+        kwargs.setdefault('form', self.base_form)
+        return super(PolymorphicChildModelAdmin, self).get_form(request, obj, **kwargs)
+
+
     def get_subclass_fields(self, request, obj=None):
         # Find out how many fields would really be on the form,
         # if it weren't restricted by declared fields.
@@ -487,7 +492,7 @@ class PolymorphicChildModelAdmin(admin.ModelAdmin):
 
         # By not declaring the fields/form in the base class,
         # get_form() will populate the form with all available fields.
-        form = self.get_form(request, obj, exclude=exclude)
+        form = self.get_form(request, obj, exclude=exclude, form=self.base_form)
         subclass_fields = list(six.iterkeys(form.base_fields)) + list(self.get_readonly_fields(request, obj))
 
         # Find which fields are not part of the common fields.