Single theme gives error

[metbril]

When I use a single theme without the selector, the build fails.

hugo.toml

theme = "hugo-classless"
[params]
theme = "https://cdn.jsdelivr.net/npm/water.css@2/out/water.css"

error:

Start building sites … 
hugo v0.151.0+extended+withdeploy darwin/arm64 BuildDate=2025-10-02T13:30:36Z VendorInfo=brew

ERROR render of "/Users/REDACTED/Documents/Code/REDACTEDl/src/content/blog/test.md" failed: "/Users/REDACTED/Documents/Code/REDACTED/src/themes/hugo-classless/layouts/baseof.html:17:60": execute of template failed: template: single.html:17:60: executing "single.html" at <toCSS (dict "minify" true)>: error calling toCSS: no Resource provided in transformation

When using the local css theme = "css/classless.css" everything is fine.

When using a list, everything is also working fine:

theme = "hugo-classless"

[[params.themes]]
name = "Water.css"
url = "https://cdn.jsdelivr.net/npm/water.css@2/out/water.css"