Weird error for broken syntax highlighting

Description of the problem or steps to reproduce

I have this btldr.yaml syntax file:

filetype: btldr
detect:
  filename: \\.btldr$
  header: "^# "

rules:
  - comment.line:
      start: "^# "
      end: $
      rules:
        - identifier.callable: "[^ ]+"

  - comment.line:
      start: "^> "
      end: $
      rules:
        - type.keyword: "\\b(Aliases|See also|More information)\\b"
        - constant.string.url: "\\bhttps://.+\\b"

  - comment.line:
      start: "^- "
      end: $
      rules:
        - identifier.type:
            start: "\\["
            end: "\\]"
            skip: "\\\\[\\[\\]]"
        - identifier.builtin: "\bstd(in|out|err)\b"

But when I start micro with *.btldr file I obtain this error and I don't know how to fix issue:

Error parsing syntax file btldr: interface conversion: interface {} is nil, not []interface {}

My goal is to highlight brackets and everything inside them in lines beginning with dash. Even when I remove skip key micro still fails.

Specifications

Commit hash:
OS:
Terminal:

Reproduce the parser error with the provided btldr.yaml syntax file and a matching *.btldr file, then inspect how micro loads syntax definitions and reports this interface-conversion failure. Done means the syntax file loads without an error and brackets and their contents are highlighted on dash-prefixed lines.