diff --git a/lib/memoist.rb b/lib/memoist.rb
index 54b67e6..fcd3141 100644
--- a/lib/memoist.rb
+++ b/lib/memoist.rb
@@ -38,10 +38,12 @@ def self.unmemoized_prefix(identifier = nil)
   end
 
   def self.escape_punctuation(string)
-    string = string.is_a?(String) ? string.dup : string.to_s
+    string = string.to_s
 
     return string unless string.end_with?('?'.freeze, '!'.freeze)
 
+    string = string.dup if string.frozen?
+
     # A String can't end in both ? and !
     if string.sub!(/\?\Z/, '_query'.freeze)
     else