Prawn::Errors::UnknownFont error occured: "is not a known font"

[hidakatsuya]

Steps to reproduce

Schema (example.tlf)

    :
  "items": [
    {
      "id": "text",
      "type": "text-block",
        :
      "style": {
        "font-family": [
          "Helvetica"
        ],
    }
  ],
    :

Code

page.item(:text).style(:bold, true).value('aaaaあ')
page.item(:text).style(:italic, true).value('aaaaあ')

Error

8: from /Users/--/.anyenv/envs/rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/prawn-2.2.2/lib/prawn/text/formatted/line_wrap.rb:77:in `apply_font_settings_and_add_fragment_to_line'
7: from /Users/--/.anyenv/envs/rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/prawn-2.2.2/lib/prawn/text/formatted/arranger.rb:166:in `apply_font_settings'
6: from /Users/--/.anyenv/envs/rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/pdf-core-0.7.0/lib/pdf/core/text.rb:214:in `character_spacing'
5: from /Users/--/.anyenv/envs/rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/prawn-2.2.2/lib/prawn/text/formatted/arranger.rb:169:in `block in apply_font_settings'
4: from /Users/--/.anyenv/envs/rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/prawn-2.2.2/lib/prawn/font.rb:57:in `font'
3: from /Users/--/.anyenv/envs/rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/prawn-2.2.2/lib/prawn/font.rb:253:in `find_font'
2: from /Users/--/.anyenv/envs/rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/prawn-2.2.2/lib/prawn/font.rb:301:in `load'
1: from /Users/--/.anyenv/envs/rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/prawn-2.2.2/lib/prawn/font.rb:301:in `new'
/Users/--/.anyenv/envs/rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/prawn-2.2.2/lib/prawn/font/afm.rb:53:in `initialize':  is not a known font. (Prawn::Errors::UnknownFont)

Environments

• Thinreports Generator 0.10.1
• Thinreports Editor 0.10.0
• Ruby 2.5.0

Cause

1. Helvetica (AFM) does not support "あ" character
2. Prawn will apply fallback font
3. IPAMincho has applied as fallback font
4. IPAMincho supports "あ" character, but does not have .ttf for bold style
5. Prawn can't find font, and raises Prawn::Errors::UnknownFont

How to Fix

• All built-in fonts also have a TTF(font file) for bold/italic style

Known Issue

In the above case, specified bold/italic are NOT applied.