Index: wp-includes/class-simplepie.php
===================================================================
--- wp-includes/class-simplepie.php	(revision 12249)
+++ wp-includes/class-simplepie.php	(working copy)
@@ -9411,6 +9411,10 @@
 		$input = SimplePie_Misc::encoding($input);
 		$output = SimplePie_Misc::encoding($output);
 
+		// Nothing to change?
+		if ( $input == $output )
+			return $data;
+
 		// We fail to fail on non US-ASCII bytes
 		if ($input === 'US-ASCII')
 		{