diff --git a/Classes/PHPExcel/Shared/String.php b/Classes/PHPExcel/Shared/String.php
index 3ba1be26..6fe9eb96 100644
--- a/Classes/PHPExcel/Shared/String.php
+++ b/Classes/PHPExcel/Shared/String.php
@@ -403,7 +403,7 @@ class PHPExcel_Shared_String
 	 * @return boolean
 	 */
 	public static function IsUTF8($value = '') {
-		return utf8_encode(utf8_decode($value)) === $value;
+		return $string === '' || preg_match('/^./su', $string) === 1;
 	}
 
 	/**