Index: wp-includes/deprecated.php
===================================================================
--- wp-includes/deprecated.php	(revision 23482)
+++ wp-includes/deprecated.php	(working copy)
@@ -3381,5 +3381,6 @@
 	$log   = log( $bytes, 1024 );
 	$power = (int) $log;
 	$size  = pow( 1024, $log - $power );
-	return $size . $units[$power];
+	$unit = array_key_exists( $power, $units ) ? $units[ $power ] : $units[0];
+	return $size . $unit;
 }