nginx 1.12.2

I have a location rule like this to proxy external images:

location /imagesproxy {
resolver 8.8.8.8;
proxy_pass $arg_uri;
proxy_pass_request_headers off;
}

The problem is that nginx doesn't seems to decode the URI

/imagesproxy?uri=http%3A%2F%2Fwww.example.com%2Fwp-content%2Fuploads%2F2015%2F07%2Fportada-otra-vez-tu-667x1000.png

error.log:
[error] 16159#16159: *3602114 invalid URL prefix in "http%3A%2F%2Fwww.alicekellen.com%2F

Any ideas?

Thank you!

-m