NGINX: invalid number of arguments in "proxy_set_header" while configuring websockets

I am trying to configure nginx for websockets.
But it returns an error.

[emerg] 8#8: invalid number of arguments in "proxy_set_header" directive in /etc/nginx/conf.d/default.conf:45
nginx: [emerg] directive in /etc/nginx/conf.d/default.conf:45

Error occurs in line 45, that's:
proxy_set_header Upgrade $http_upgrade;

I have no idea why it doesn't work, I copied it from nginx docs.

Looking for solutions I tried adding a \ before the $, switching nginx version. <br/>
It didn't help.

location /ws/ {
        proxy_pass http://backend;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
    }

default.conf - Full config file

upstream frontend {
    server frontend:3000;
}

upstream backend {
    server backend:8000;
}

server {
    listen 80;
    listen [::]:80;
    server_name ${DOMAIN} www.${DOMAIN};

    location /.well-known/acme-challenge/ {
        root /vol/www/;
    }

    location / {
        return 301 https://${DOMAIN}$request_uri;
    }
}

server {
    listen [::]:443 ssl ipv6only=on;
    listen 443 ssl;
    server_name ${DOMAIN} www.${DOMAIN};

    ssl_certificate     /etc/letsencrypt/live/${DOMAIN}/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/${DOMAIN}/privkey.pem;

    include     /etc/nginx/options-ssl-nginx.conf;

    ssl_dhparam /vol/proxy/ssl-dhparams.pem;

    add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;

    location /api/ {
        proxy_pass http://backend;
        proxy_redirect off;
    }

    location /ws/ {
        proxy_pass http://backend;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
    }

    location / {
        proxy_pass           http://frontend;
        proxy_redirect       off;
        client_max_body_size 10M;
    }

    error_page 500 502 503 504 /50x.html;

    location = /50x.html {
        root /usr/share/nginx/html;
    }
}

Any idea what I should change to make it work?

I thought I could put proxy_set_header into another file, then use include. <br/>
It works.
I created a file called proxy_params, in which I put the following

proxy_params

proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";

Then in the configuration file I used

include /etc/nginx/proxy_params;

<br/>

This is what my config for websockets looks like

default.conf - websocket config

location /ws/ {
    proxy_pass http://backend;
    proxy_http_version 1.1;
    include /etc/nginx/proxy_params;
}