Index: wp-includes/deprecated.php
===================================================================
--- wp-includes/deprecated.php	(revision 23502)
+++ wp-includes/deprecated.php	(working copy)
@@ -3381,9 +3381,9 @@
 	$units = array( 0 => 'B', 1 => 'kB', 2 => 'MB', 3 => 'GB' );
 	$log   = log( $bytes, 1024 );
 	$power = (int) $log;
+	$size  = pow( 1024, $log - $power );
 
-	if ( array_key_exists( $power, $units ) ) {
-		$size  = pow( 1024, $log - $power );
+	if ( ! is_nan( $size ) && array_key_exists( $power, $units ) ) {
 		$unit = $units[ $power ];
 	} else {
 		$size = $bytes;