diff --git a/judge/admin/profile.py b/judge/admin/profile.py
index 2bd3ae9..4c380c9 100644
--- a/judge/admin/profile.py
+++ b/judge/admin/profile.py
@@ -4,6 +4,7 @@ from django.utils.html import format_html
 from django.utils.translation import gettext, gettext_lazy as _, ungettext
 from django.contrib.auth.admin import UserAdmin as OldUserAdmin
 from django.core.exceptions import ValidationError
+from django.contrib.auth.forms import UserChangeForm
 
 from django_ace import AceWidget
 
@@ -171,12 +172,12 @@ class ProfileAdmin(VersionAdmin):
     recalculate_points.short_description = _("Recalculate scores")
 
 
-class UserForm(ModelForm):
-    username = CharField(
-        max_length=150,
-        help_text=_("Username can only contain letters, digits, and underscores."),
-        widget=TextInput(attrs={"class": "vTextField"}),
-    )
+class UserForm(UserChangeForm):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.fields["username"].help_text = _(
+            "Username can only contain letters, digits, and underscores."
+        )
 
     def clean_username(self):
         username = self.cleaned_data.get("username")
@@ -221,3 +222,6 @@ class UserAdmin(OldUserAdmin):
                 "user_permissions",
             )
         return fields
+
+    def has_add_permission(self, request):
+        return False
diff --git a/judge/views/comment.py b/judge/views/comment.py
index 73506bf..8ac62fa 100644
--- a/judge/views/comment.py
+++ b/judge/views/comment.py
@@ -218,13 +218,18 @@ class CommentRevisionAjax(CommentMixin, DetailView):
     def get_context_data(self, **kwargs):
         context = super(CommentRevisionAjax, self).get_context_data(**kwargs)
         revisions = Version.objects.get_for_object(self.object).order_by("-revision")
+
+        if len(revisions) == 0:
+            raise Http404
+
         try:
             wanted = min(
                 max(int(self.request.GET.get("revision", 0)), 0), len(revisions) - 1
             )
-        except ValueError:
+            revision = revisions[wanted]
+        except (ValueError, IndexError):
             raise Http404
-        revision = revisions[wanted]
+
         data = json.loads(revision.serialized_data)
         try:
             context["body"] = data[0]["fields"]["body"]