diff --git a/lib/httparty.rb b/lib/httparty.rb
index 4a5a74a..0a6100a 100644
--- a/lib/httparty.rb
+++ b/lib/httparty.rb
@@ -105,7 +105,7 @@ module HTTParty
     #     format :json
     #   end
     def format(f)
-      raise UnsupportedFormat, "Must be one of: #{AllowedFormats.values.map(&:to_s).uniq.sort.join(', ')}" unless AllowedFormats.value?(f)
+      raise UnsupportedFormat, "Must be one of: #{AllowedFormats.values.map { |v| v.to_s }.uniq.sort.join(', ')}" unless AllowedFormats.value?(f)
       default_options[:format] = f
     end