这是最新版本dev中的错误.根据此答案.该错误在任何稳定版本中均不存在.将您的composer.json的minimum-stability更改为stable并运行作曲家更新. Github上此线程的更多信息.

It's a bug in the latest dev version. According to this answer. The bug is not present on any stable release. Change your composer.json's minimum-stability to stable and run a composer update. More on this thread on Github.