Index: tag.php
===================================================================
--- tag.php	(revision 3884)
+++ tag.php	(working copy)
@@ -79,7 +79,10 @@
 	public static function rewrite_tag_exists($rule, $slug, $parameters)
 	{
 		$tag = Tag::get($rule->named_arg_values['tag']);
-		return $tag instanceOf Tag;
+		if ( !$tag instanceOf Tag ) {
+			return FALSE;
+		}
+		return Posts::count_by_tag($tag->tag_text, Post::status('published')) > 0;
 	}
 
 	/**