fatal: Could not change back to '/root': Permission denied

I found a bug but I don't really know why it happens.

I logged in as root, so in /root. Then I typed "su my_user" and I tried to "fundle install" but I was greeted with:

fatal: Could not change back to '/root': Permission denied

So I went to ~ and installed fine.

Then, after installation, I did the same thing, I logged in as root, so in /root, Then I typed "su my_user" and error:

```
find: '.': Permission denied
find: Failed to restore initial working directory: /root: Permission denied
```
Mind explaining why?

It only happens in /root, not in "/", not in "/etc"