weird behaviour about cannot complete a directory which it can be `ls`

[Wu Zhenyu <wuzhenyu@xxxxxxxx>]

In Android Termux, I try:

```zsh
% ls /<TAB>
```

Nothing is completed. In fact:

```zsh
% ls /
"/": Permission denied (os error 13)
```

However, I can:

```zsh
% ls /sdcard
Android ...
```

The weird thing is:

```zsh
% ls /sdcard/<TAB>
```

Also nothing is completed.

Why the weird thing happen? How can I make `ls /sdcard/<TAB>` work?

TIA!

-- 
Best Regards
Wu Zhenyu