Error building site: failed to render pages: render of "page" failed: execute of template failed

[bahmanrostamitabar]

I am trying to create a new website with default content but am getting following error after running blogdown::serve_site():

Launching the server via the command:
/Users/bahmanrostami-tabar/Library/Application Support/Hugo/0.80.0/hugo server --bind 127.0.0.1 -p 4321 --themesDir themes -t hugo-cards -D -F --navigateToChanged
Error: Error building site: failed to render pages: render of "page" failed: execute of template failed: template: pages/single.html:4:5: executing "pages/single.html" at <partial "head.html" .>: error calling partial: "/Users/bahmanrostami-tabar/Documents/3.Teaching websites/test/themes/hugo-cards/layouts/partials/head.html:12:14": execute of template failed: template: partials/head.html:12:14: executing "partials/head.html" at <.Site.Params.custom_css>: range can't iterate over css/custom.css

blogdown::hugo_version()
[1] ‘0.80.0’

packageVersion("blogdown")
[1] ‘1.3’