diff --git a/wp-includes/canonical.php b/wp-includes/canonical.php
index ff3693791..978d988ba 100644
--- a/wp-includes/canonical.php
+++ b/wp-includes/canonical.php
@@ -34,10 +34,10 @@
  * @return null|false|string Null, if redirect not needed. False, if redirect
  *		not needed or the string of the URL
  */
-function redirect_canonical($requested_url=null, $do_redirect=true) {
+function redirect_canonical( $requested_url = null, $do_redirect = true ) {
 	global $wp_rewrite, $is_IIS, $wp_query, $wpdb;
 
-	if ( is_trackback() || is_search() || is_comments_popup() || is_admin() || $is_IIS || ( isset($_POST) && count($_POST) ) || is_preview() || is_robots() )
+	if ( is_trackback() || is_search() || is_comments_popup() || is_admin() || !empty($_POST) || is_preview() || is_robots() || ( $is_IIS && !iis7_supports_permalinks() ) )
 		return;
 
 	if ( !$requested_url ) {