diff --git a/src/applications/settings/panel/PhabricatorSettingsPanelDisplayPreferences.php b/src/applications/settings/panel/PhabricatorSettingsPanelDisplayPreferences.php
--- a/src/applications/settings/panel/PhabricatorSettingsPanelDisplayPreferences.php
+++ b/src/applications/settings/panel/PhabricatorSettingsPanelDisplayPreferences.php
@@ -32,7 +32,7 @@
       $monospaced = $request->getStr($pref_monospaced);
 
       // Prevent the user from doing stupid things.
-      $monospaced = preg_replace('/[^a-z0-9 ,"]+/i', '', $monospaced);
+      $monospaced = preg_replace('/[^a-z0-9 ,".]+/i', '', $monospaced);
 
       $preferences->setPreference($pref_titles, $request->getStr($pref_titles));
       $preferences->setPreference($pref_editor, $request->getStr($pref_editor));