diff --git a/spec/prawn/font_spec.rb b/spec/prawn/font_spec.rb
index a890c907e..39326c3bf 100644
--- a/spec/prawn/font_spec.rb
+++ b/spec/prawn/font_spec.rb
@@ -670,4 +670,42 @@ def page_should_not_include_font(font)
     font_name = text.font_settings.first[:name].to_s.sub(/\w+\+/, 'subset+')
     expect(font_name).to eq 'subset+DustismoRoman'
   end
+
+  it 'does not change width of unknown glyph' do
+    text_with_string_widths = Class.new(PDF::Inspector::Text) do
+      attr_reader :string_widths
+
+      def initialize(*)
+        super
+        @string_widths = []
+      end
+
+      def show_text text, kerned = false
+        super
+        @string_widths << ((@state.current_font.unpack text).reduce(0) do |width, code|
+          width + (@state.current_font.glyph_width code) * @font_settings[-1][:size] / 1000.0
+        end)
+      end
+    end
+
+    pdf = Prawn::Document.new do
+      font_families.update(
+        'DejaVu Sans' => {
+          normal: "#{Prawn::DATADIR}/fonts/DejaVuSans.ttf",
+          bold: "#{Prawn::DATADIR}/fonts/DejaVuSans-Bold.ttf"
+        },
+      )
+
+      # changing option to subset: false fixes issue (albeit using different behavior)
+      font 'DejaVu Sans', subset: true do
+        text '日本語<b>end</b>', inline_format: true
+      end
+    end
+
+    rendered_pdf = pdf.render
+    #File.write '/tmp/debug.pdf', rendered_pdf
+    analyzed_pdf = text_with_string_widths.analyze(rendered_pdf)
+    expect(analyzed_pdf.string_widths.length).to eql(2)
+    expect(analyzed_pdf.string_widths[0]).to be > 0.0
+  end
 end