diff --git a/lib/prawn/fonts/ttf.rb b/lib/prawn/fonts/ttf.rb
index 5e55220f9..779234de8 100644
--- a/lib/prawn/fonts/ttf.rb
+++ b/lib/prawn/fonts/ttf.rb
@@ -393,18 +393,18 @@ def embed_simple_font(reference, font, unicode_mapping)
           XHeight: x_height
         )
 
-        first_char = font.cmap.tables.first.code_map.index { |gid| !gid.zero? }
-        last_char = font.cmap.tables.first.code_map.rindex { |gid| !gid.zero? }
+        first_char, last_char = unicode_mapping.keys.minmax
         hmtx = font.horizontal_metrics
         widths =
-          font.cmap.tables.first.code_map[first_char..last_char].map do |gid|
-            if gid.zero?
+          (first_char..last_char).map do |code|
+            if unicode_mapping.key?(code)
+              gid = font.cmap.tables.first.code_map[code]
+              Integer(hmtx.widths[gid] * scale_factor)
+            else
               # These characters are not in the document so we don't ever use
               # these values but we need to encode them so let's use as little
               # sapce as possible.
               0
-            else
-              Integer(hmtx.widths[gid] * scale_factor)
             end
           end
 
diff --git a/spec/prawn_manual_spec.rb b/spec/prawn_manual_spec.rb
index b92c55441..105016409 100644
--- a/spec/prawn_manual_spec.rb
+++ b/spec/prawn_manual_spec.rb
@@ -6,9 +6,9 @@
 MANUAL_HASH =
   case RUBY_ENGINE
   when 'ruby'
-    '8ace5f35f945e5994647cefc2cf7bc369d131e0646d91eb8aeb94e58f72de18d8e7bf82f58fc45406110c4adad239dcbe834059580d29fec2b2a039db67db04c'
+    '7991e4f72e944140840e1c26f0fff331029846eaab148de8483d06491c7808bc4963e8e7376a514e855037f1f1b4197877a31f2df44f511f4f7f5e0ce5df3170'
   when 'jruby'
-    'b77a740d3290192360c4c083018ca61ccc88d42e7c6a152c7bc394d751f5f08d85aec613549f6660713644b00518561cc0f9c947701f01e8c25632c9db81201a'
+    '29b8f8cb00910426805ce226fb47c59d6409683f35f0d2c056a6cf837ba086ca5c763ff89266cfc8e11b1d92af60c9974822b12ad761cdbdf520adb005a98750'
   end
 
 RSpec.describe Prawn do