PHPUnit-致命错误:当测试类为空时，方法PHPUnit_Framework_AssertionFailedErro

我刚刚开始学习PHPUnit，当我创建一个像这样的空类时：

<?php
namespace some'ns;
use some'ns'AClass;
/**
 * Test class for the some'ns'AClass class
 */
class AClassTest extends 'PHPUnit_Framework_TestCase {
}

当我运行测试时：

$ phpunit -c .
PHPUnit 4.3.5 by Sebastian Bergmann.
Configuration read from path/to/phpunit.xml.dist
F
Time: 281 ms, Memory: 3.50Mb
There was 1 failure:
1) Warning
Fatal error: Method PHPUnit_Framework_AssertionFailedError::__toString() must not throw an exception in phar:///usr/local/bin/phpunit/phpunit/TextUI/ResultPrinter.php on line 0
Call Stack:
    0.0329     339512   1. {main}() /usr/local/bin/phpunit:0
    0.0330     339512   2. {main}() /usr/local/bin/phpunit:0
    0.1409     779176   3. PHPUnit_TextUI_Command::main() /usr/local/bin/phpunit:605
    0.1409     779424   4. PHPUnit_TextUI_Command::main() /usr/local/bin/phpunit:605
    0.1409     780048   5. PHPUnit_TextUI_Command->run() phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:138
    0.1410     780544   6. PHPUnit_TextUI_Command->run() phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:138
    0.2650    2779792   7. PHPUnit_TextUI_TestRunner->doRun() phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:186
    0.2650    2780016   8. PHPUnit_TextUI_TestRunner->doRun() phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:186
    0.2806    3526000   9. PHPUnit_TextUI_ResultPrinter->printResult() phar:///usr/local/bin/phpunit/phpunit/TextUI/TestRunner.php:429
    0.2806    3526360  10. PHPUnit_TextUI_ResultPrinter->printResult() phar:///usr/local/bin/phpunit/phpunit/TextUI/TestRunner.php:429
    0.2808    3529928  11. PHPUnit_TextUI_ResultPrinter->printFailures() phar:///usr/local/bin/phpunit/phpunit/TextUI/ResultPrinter.php:177
    0.2808    3530160  12. PHPUnit_TextUI_ResultPrinter->printFailures() phar:///usr/local/bin/phpunit/phpunit/TextUI/ResultPrinter.php:177
    0.2808    3530464  13. PHPUnit_TextUI_ResultPrinter->printDefects() phar:///usr/local/bin/phpunit/phpunit/TextUI/ResultPrinter.php:288
    0.2808    3530464  14. PHPUnit_TextUI_ResultPrinter->printDefects() phar:///usr/local/bin/phpunit/phpunit/TextUI/ResultPrinter.php:288
    0.2808    3530960  15. PHPUnit_TextUI_ResultPrinter->printDefect() phar:///usr/local/bin/phpunit/phpunit/TextUI/ResultPrinter.php:233
    0.2808    3531192  16. PHPUnit_TextUI_ResultPrinter->printDefect() phar:///usr/local/bin/phpunit/phpunit/TextUI/ResultPrinter.php:233
    0.2809    3532280  17. PHPUnit_TextUI_ResultPrinter->printDefectTrace() phar:///usr/local/bin/phpunit/phpunit/TextUI/ResultPrinter.php:244
    0.2809    3532512  18. PHPUnit_TextUI_ResultPrinter->printDefectTrace() phar:///usr/local/bin/phpunit/phpunit/TextUI/ResultPrinter.php:244

引发此异常。出于好奇，有人能告诉我为什么吗？