--- lib/prawn/font.rb.orig	2018-11-08 20:45:32.309671191 +0000
+++ lib/prawn/font.rb	2018-11-08 20:46:17.537133702 +0000
@@ -384,7 +384,7 @@
     end
 
     def identifier_for(subset) #:nodoc:
-      "#{@identifier}.#{subset}"
+      "#{@identifier}.#{subset}".to_sym
     end
 
     def inspect #:nodoc: