Frozen works strangely on nils

middleman-core/lib/middleman-core/contracts.rb

@@ -23,7 +23,7 @@ if ENV['TEST'] || ENV['CONTRACTS'] == 'true'
       end
 
       def valid?(val)
-        val.frozen? && Contract.valid?(val, @contract)
+        (val.frozen? || val.nil?) && Contract.valid?(val, @contract)
       end
     end
 

middleman-core/lib/middleman-core/util/hash_with_indifferent_access.rb

@@ -92,7 +92,7 @@ module Middleman
           self.class.new(data)
         elsif data.is_a? Array
           data.map(&method(:recursively_enhance)).freeze
-        elsif data.frozen?
+        elsif data.frozen? || data.nil?
           data
         else
           data.dup.freeze