Mattermost configuration/integration via reverse proxy - help wanted

I am using reverse proxy and hosting gitlab on vlabs.iitb.ac.in/gitlab/
i have updated to 9.0 and trying to integrate mattermost.

in my gitlab.rb file, i activated the mattermost line and changed the URL
from

mattermost_external_url ‘http://mattermost.example.com

to
mattermost_external_url ‘http://vlabs.iitb.ac.in/mattermost

(i am hosting gitlab with similar external url settings
external_url ‘http://vlabs.iitb.ac.in/gitlab’ )
after the reconfigure, i get the following error. cant understand where i am going wrong.
I want to host mattermost and gitlab on same server
Seeking your help buddies…

Starting Chef Client, version 12.12.15
resolving cookbooks for run list: [“gitlab”]
Synchronizing Cookbooks:

  • package (0.0.0)
  • gitlab (0.0.1)
  • runit (0.14.2)
    Installing Cookbook Gems:
    Compiling Cookbooks…

================================================================================
Recipe Compile Error in /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/recipes/default.rb

RuntimeError

Unsupported CI external URL path: /mattermost

Cookbook Trace:

/opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/libraries/gitlab_mattermost.rb:62:in parse_mattermost_external_url' /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/libraries/gitlab_mattermost.rb:23:in parse_variables’
/opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/libraries/gitlab.rb:238:in generate_config' /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/recipes/config.rb:24:in from_file’
/opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/recipes/default.rb:26:in `from_file’

Relevant File Content:

/opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/libraries/gitlab_mattermost.rb:

55: Gitlab[‘mattermost_nginx’][‘ssl_certificate_key’] ||= “/etc/gitlab/ssl/#{uri.host}.key”
56: Nginx.parse_proxy_headers(‘mattermost_nginx’, true)
57: else
58: raise “Unsupported external URL scheme: #{uri.scheme}”
59: end
60:
61: unless ["", “/”].include?(uri.path)
62>> raise “Unsupported CI external URL path: #{uri.path}”
63: end
64:
65: Gitlab[‘mattermost’][‘port’] = uri.port
66: end
67:
68: def parse_gitlab_mattermost
69: return unless Gitlab[‘mattermost’][‘enable’]
70:
71: Gitlab[‘mattermost_nginx’][‘enable’] = true if Gitlab[‘mattermost_nginx’][‘enable’].nil?

Platform:

x86_64-linux

Running handlers:
Running handlers complete
Chef Client failed. 0 resources updated in 06 seconds