diff --git a/middleman-core/lib/middleman-core/contracts.rb b/middleman-core/lib/middleman-core/contracts.rb
index 2f13bc1c..5fd05e11 100644
--- a/middleman-core/lib/middleman-core/contracts.rb
+++ b/middleman-core/lib/middleman-core/contracts.rb
@@ -17,6 +17,16 @@ if ENV['TEST'] || ENV['CONTRACTS'] == 'true'
       end
     end
 
+    class Frozen < CallableClass
+      def initialize(contract)
+        @contract = contract
+      end
+
+      def valid?(val)
+        val.frozen? && Contract.valid?(val, @contract)
+      end
+    end
+
     class ArrayOf
       def initialize(contract)
         @contract = contract.is_a?(String) ? IsA[contract] : contract
@@ -128,6 +138,9 @@ else
 
     class SetOf < Callable
     end
+
+    class Frozen < Callable
+    end
   end
 end
 
diff --git a/middleman-core/lib/middleman-core/core_extensions/data.rb b/middleman-core/lib/middleman-core/core_extensions/data.rb
index 95fabdcd..e5824362 100644
--- a/middleman-core/lib/middleman-core/core_extensions/data.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/data.rb
@@ -97,11 +97,11 @@ module Middleman
 
           path = data_path.to_s.split(File::SEPARATOR)[0..-2]
           path.each do |dir|
-            data_branch[dir] ||= ::Middleman::Util.recursively_enhance({})
+            data_branch[dir] ||= {}
             data_branch = data_branch[dir]
           end
 
-          data_branch[basename] = data && ::Middleman::Util.recursively_enhance(data)
+          data_branch[basename] = data
         end
 
         # Remove a given file from the internal cache
@@ -132,14 +132,13 @@ module Middleman
         # @return [Hash, nil]
         Contract Or[String, Symbol] => Maybe[Hash]
         def data_for_path(path)
-          response = nil
-
-          if store.key?(path.to_s)
-            response = store[path.to_s]
+          response = if store.key?(path.to_s)
+            store[path.to_s]
           elsif callbacks.key?(path.to_s)
-            response = callbacks[path.to_s].call
+            callbacks[path.to_s].call
           end
 
+          response = ::Middleman::Util.recursively_enhance(response)
           response
         end
 
@@ -149,10 +148,11 @@ module Middleman
         # @return [Hash, nil]
         def method_missing(path)
           if @local_data.key?(path.to_s)
+            @local_data[path.to_s] = ::Middleman::Util.recursively_enhance(@local_data[path.to_s])
             return @local_data[path.to_s]
           else
             result = data_for_path(path)
-            return ::Middleman::Util.recursively_enhance(result) if result
+            return result if result
           end
 
           super
diff --git a/middleman-core/lib/middleman-core/file_renderer.rb b/middleman-core/lib/middleman-core/file_renderer.rb
index 6e43d712..a97eae7c 100644
--- a/middleman-core/lib/middleman-core/file_renderer.rb
+++ b/middleman-core/lib/middleman-core/file_renderer.rb
@@ -51,7 +51,7 @@ module Middleman
 
       # Merge per-extension options from config
       extension = File.extname(path)
-      options = opts.dup.merge(options_for_ext(extension))
+      options = opts.merge(options_for_ext(extension))
       options[:outvar] ||= '@_out_buf'
       options.delete(:layout)
 
diff --git a/middleman-core/lib/middleman-core/preview_server.rb b/middleman-core/lib/middleman-core/preview_server.rb
index c0d8f7c2..82b65acc 100644
--- a/middleman-core/lib/middleman-core/preview_server.rb
+++ b/middleman-core/lib/middleman-core/preview_server.rb
@@ -17,7 +17,7 @@ module Middleman
       # Start an instance of Middleman::Application
       # @return [void]
       def start(opts={})
-        @options = opts
+        @options = opts.dup.freeze
         @host = @options[:host] || '0.0.0.0'
         @port = @options[:port] || DEFAULT_PORT
 
@@ -94,7 +94,7 @@ module Middleman
       private
 
       def new_app
-        opts = @options.dup
+        opts = @options
 
         ::Middleman::Logger.singleton(
           opts[:debug] ? 0 : 1,
diff --git a/middleman-core/lib/middleman-core/sitemap/resource.rb b/middleman-core/lib/middleman-core/sitemap/resource.rb
index add3919d..5f9ecc67 100644
--- a/middleman-core/lib/middleman-core/sitemap/resource.rb
+++ b/middleman-core/lib/middleman-core/sitemap/resource.rb
@@ -79,7 +79,7 @@ module Middleman
       Contract None => IsA['Middleman::Util::HashWithIndifferentAccess']
       def data
         # TODO: Should this really be a HashWithIndifferentAccess?
-        ::Middleman::Util.recursively_enhance(metadata[:page]).freeze
+        ::Middleman::Util.recursively_enhance(metadata[:page])
       end
 
       # Options about how this resource is rendered, such as its :layout,
diff --git a/middleman-core/lib/middleman-core/template_context.rb b/middleman-core/lib/middleman-core/template_context.rb
index 78becaf5..1cca89aa 100644
--- a/middleman-core/lib/middleman-core/template_context.rb
+++ b/middleman-core/lib/middleman-core/template_context.rb
@@ -30,10 +30,10 @@ module Middleman
     # @param [Middleman::Application] app
     # @param [Hash] locs
     # @param [Hash] opts
-    def initialize(app, locs={}, opts={})
+    def initialize(app, locs={}.freeze, opts={}.freeze)
       @app = app
-      @locs = locs.dup.freeze
-      @opts = opts.dup.freeze
+      @locs = locs
+      @opts = opts
     end
 
     # Return the current buffer to the caller and clear the value internally.
diff --git a/middleman-core/lib/middleman-core/template_renderer.rb b/middleman-core/lib/middleman-core/template_renderer.rb
index 0eb2fc8f..72fa9db2 100644
--- a/middleman-core/lib/middleman-core/template_renderer.rb
+++ b/middleman-core/lib/middleman-core/template_renderer.rb
@@ -31,16 +31,19 @@ module Middleman
     Contract Hash, Hash => String
     def render(locs={}, opts={})
       path = @path.dup
+      locals = locs.dup.freeze
+      options = opts.dup
+
       extension = File.extname(path)
       engine = extension[1..-1].to_sym
 
       if defined?(::I18n)
         old_locale = ::I18n.locale
-        ::I18n.locale = opts[:lang] if opts[:lang]
+        ::I18n.locale = options[:lang] if options[:lang]
       end
 
       # Sandboxed class for template eval
-      context = @app.template_context_class.new(@app, locs, opts)
+      context = @app.template_context_class.new(@app, locals, options)
 
       # TODO: Only for HAML files
       context.init_haml_helpers if context.respond_to?(:init_haml_helpers)
@@ -50,10 +53,10 @@ module Middleman
       content = nil
       while ::Tilt[path]
         begin
-          opts[:template_body] = content if content
+          options[:template_body] = content if content
 
           content_renderer = ::Middleman::FileRenderer.new(@app, path)
-          content = content_renderer.render(locs, opts, context)
+          content = content_renderer.render(locals, options, context)
 
           path = File.basename(path, File.extname(path))
         rescue LocalJumpError
@@ -62,9 +65,9 @@ module Middleman
       end
 
       # If we need a layout and have a layout, use it
-      if layout_path = fetch_layout(engine, opts)
+      if layout_path = fetch_layout(engine, options)
         layout_renderer = ::Middleman::FileRenderer.new(@app, layout_path)
-        content = layout_renderer.render(locs, opts, context) { content }
+        content = layout_renderer.render(locals, options, context) { content }
       end
 
       # Return result
diff --git a/middleman-core/lib/middleman-core/util.rb b/middleman-core/lib/middleman-core/util.rb
index 7556b504..5ce84523 100644
--- a/middleman-core/lib/middleman-core/util.rb
+++ b/middleman-core/lib/middleman-core/util.rb
@@ -15,11 +15,11 @@ require 'rack/mime'
 require 'middleman-core/contracts'
 
 module Middleman
-
   module Util
-    extend self
     include Contracts
 
+    module_function
+
     # Whether the source file is binary.
     #
     # @param [String] filename The file to check.
@@ -74,32 +74,16 @@ module Middleman
     # @private
     # @param [Hash] data Normal hash
     # @return [Middleman::Util::HashWithIndifferentAccess]
-    Contract Or[Hash, Array] => Or[HashWithIndifferentAccess, Array]
+    Contract Maybe[Or[Array, Hash, HashWithIndifferentAccess]] => Maybe[Frozen[Or[HashWithIndifferentAccess, Array]]]
     def recursively_enhance(data)
-      if data.is_a? Hash
-        enhanced = ::Middleman::Util::HashWithIndifferentAccess.new(data)
-
-        enhanced.each do |key, val|
-          enhanced[key] = if val.is_a?(Hash) || val.is_a?(Array)
-            recursively_enhance(val)
-          else
-            val
-          end
-        end
-
-        enhanced
+      if data.is_a? HashWithIndifferentAccess
+        data
+      elsif data.is_a? Hash
+        HashWithIndifferentAccess.new(data)
       elsif data.is_a? Array
-        enhanced = data.dup
-
-        enhanced.each_with_index do |val, i|
-          enhanced[i] = if val.is_a?(Hash) || val.is_a?(Array)
-            recursively_enhance(val)
-          else
-            val
-          end
-        end
-
-        enhanced
+        data.map(&method(:recursively_enhance))
+      else
+        nil
       end
     end
 
diff --git a/middleman-core/lib/middleman-core/util/hash_with_indifferent_access.rb b/middleman-core/lib/middleman-core/util/hash_with_indifferent_access.rb
index bae61b95..4f2254e9 100644
--- a/middleman-core/lib/middleman-core/util/hash_with_indifferent_access.rb
+++ b/middleman-core/lib/middleman-core/util/hash_with_indifferent_access.rb
@@ -1,3 +1,5 @@
+require 'middleman-core/contracts'
+
 module Middleman
   module Util
     # A hash with indifferent access and magic predicates.
@@ -10,11 +12,17 @@ module Middleman
     #   hash.foo?   #=> true
     #
     class HashWithIndifferentAccess < ::Hash #:nodoc:
+      include Contracts
+
+      Contract Hash => Any
       def initialize(hash={})
         super()
-        hash.each do |key, value|
-          self[convert_key(key)] = value
+
+        hash.each do |key, val|
+          self[key] = recursively_enhance(val)
         end
+
+        freeze
       end
 
       def [](key)
@@ -73,6 +81,23 @@ module Middleman
           self[method]
         end
       end
+
+      private
+
+      Contract Any => Frozen[Any]
+      def recursively_enhance(data)
+        if data.is_a? HashWithIndifferentAccess
+          data
+        elsif data.is_a? Hash
+          self.class.new(data)
+        elsif data.is_a? Array
+          data.map(&method(:recursively_enhance)).freeze
+        elsif data.frozen?
+          data
+        else
+          data.dup.freeze
+        end
+      end
     end
   end
 end