我使用asciidoctor-pdf将adoc文档转换为pdf，但总是出现“不是已知字体”的错误，在命令行后添加--trace时，错误是
`initialize': is not a known font. (Prawn::Errors::UnknownFont)
/usr/local/lib/ruby/gems/2.4.0/gems/prawn-2.2.2/lib/prawn/font/afm.rb:53:in `initialize': is not a known font. (Prawn::Errors::UnknownFont)
from /usr/local/lib/ruby/gems/2.4.0/gems/prawn-2.2.2/lib/prawn/font.rb:301:in `new'
from /usr/local/lib/ruby/gems/2.4.0/gems/prawn-2.2.2/lib/prawn/font.rb:301:in `load'
from /usr/local/lib/ruby/gems/2.4.0/gems/prawn-2.2.2/lib/prawn/font.rb:253:in `find_font'
from /usr/local/lib/ruby/gems/2.4.0/gems/prawn-2.2.2/lib/prawn/font.rb:57:in `font'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-pdf-1.5.0.alpha.16/lib/asciidoctor-pdf/prawn_ext/extensions.rb:196:in `font'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-pdf-1.5.0.alpha.16/lib/asciidoctor-pdf/converter.rb:2930:in `theme_font'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-pdf-1.5.0.alpha.16/lib/asciidoctor-pdf/converter.rb:544:in `convert_admonition'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-pdf-1.5.0.alpha.16/lib/asciidoctor-pdf/converter.rb:118:in `convert'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-1.5.6.1/lib/asciidoctor/abstract_block.rb:70:in `convert'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-1.5.6.1/lib/asciidoctor/abstract_block.rb:79:in `block in content'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-1.5.6.1/lib/asciidoctor/abstract_block.rb:79:in `map'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-1.5.6.1/lib/asciidoctor/abstract_block.rb:79:in `content'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-1.5.6.1/lib/asciidoctor/block.rb:110:in `content'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-pdf-1.5.0.alpha.16/lib/asciidoctor-pdf/converter.rb:134:in `convert_content_for_block'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-pdf-1.5.0.alpha.16/lib/asciidoctor-pdf/converter.rb:479:in `convert_preamble'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-pdf-1.5.0.alpha.16/lib/asciidoctor-pdf/converter.rb:118:in `convert'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-1.5.6.1/lib/asciidoctor/abstract_block.rb:70:in `convert'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-1.5.6.1/lib/asciidoctor/abstract_block.rb:79:in `block in content'
from /usr/local/lib/ruby/gems/2.4.0/gems/asciidoctor-1.5.6.1/lib/asciidoctor/abstract_block.rb:79:in `map'
from …