500 error_msg on any request

When point my browser at my local writefreely instance, I get a 500:

{"code":500,"error_msg":"This is an unhelpful error message for a miscellaneous internal error."}

In the logs I find these lines:

ERROR: 2023/05/26 07:55:51 collections.go:873: Unable to render collection index: template: collection.tmpl:9:7: executing "collection" at <.CustomCSS>: can't evaluate field CustomCSS in type writefreely.CollectionPage
ERROR: 2023/05/26 07:55:51 handle.go:533: [Web handler] 500: template: collection.tmpl:9:7: executing "collection" at <.CustomCSS>: can't evaluate field CustomCSS in type writefreely.CollectionPage
2023/05/26 07:55:51 Web internal error render
2023/05/26 07:55:51 "GET /" 500 28.99877ms "Mozilla/5.0 (X11; Linux x86_64; rv:109.0) Gecko/20100101 Firefox/113.0"
2023/05/26 07:55:51 http: superfluous response.WriteHeader call from github.com/writeas/impart.writeBody (response.go:31)

Is this a bug or did I miss something? I’m using writefreely 0.13.2.