diff --git a/plinth/modules/tiddlywiki/forms.py b/plinth/modules/tiddlywiki/forms.py
index 227aa1601..4d0af3cfb 100644
--- a/plinth/modules/tiddlywiki/forms.py
+++ b/plinth/modules/tiddlywiki/forms.py
@@ -6,12 +6,19 @@ from django.core import validators
 from django.utils.translation import gettext_lazy as _
 
 
+def _validate_not_index_file(name):
+    """Validate that the normalized file name is not 'index.html'."""
+    if str(name) in ('index.html', 'index'):
+        raise forms.ValidationError(
+            _('Wiki files cannot be named "index.html".'))
+
+
 class CreateWikiForm(forms.Form):
     """Form to create a new wiki file."""
 
     name = forms.CharField(
         label=_('Name of the wiki file, with file extension ".html"'),
-        strip=True, help_text=_(
+        strip=True, validators=[_validate_not_index_file], help_text=_(
             'Wiki title and description can be set from within the wiki. '
             'This file name is independent of the wiki title.'))
 
@@ -21,7 +28,7 @@ class RenameWikiForm(forms.Form):
 
     new_name = forms.CharField(
         label=_('New name for the wiki file, with file extension ".html"'),
-        strip=True, help_text=_(
+        strip=True, validators=[_validate_not_index_file], help_text=_(
             'Renaming the file has no effect on the title of the wiki.'))
 
 
@@ -29,9 +36,11 @@ class UploadWikiForm(forms.Form):
     """Form to upload a wiki file."""
 
     file = forms.FileField(
-        label=_('A TiddlyWiki file with .html file extension'),
-        required=True, validators=[
+        label=_('A TiddlyWiki file with .html file extension'), required=True,
+        validators=[
             validators.FileExtensionValidator(
-                ['html'], _('TiddlyWiki files must be in HTML format'))
-        ], help_text=_(
-            'Upload an existing TiddlyWiki file from this computer.'))
+                ['html'],
+                _('TiddlyWiki files must be in HTML format'),
+            ), _validate_not_index_file
+        ],
+        help_text=_('Upload an existing TiddlyWiki file from this computer.'))