autoindex directive in 'if' section

[B.R.]

Hello,

I would like to use the following in my conf file:
location a/ {
 if ($host ~* "") {
  root "";
  autoindex on;
 }
}

But that doesn't work: 'nginx: [emerg] "autoindex" directive is not allowed
here'

What should I do?

Thanks,
---
*B. R.*
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