diff --git a/lib/sass/compiler/host/function_registry.rb b/lib/sass/compiler/host/function_registry.rb
index 29e2c11..9f8786a 100644
--- a/lib/sass/compiler/host/function_registry.rb
+++ b/lib/sass/compiler/host/function_registry.rb
@@ -7,11 +7,12 @@ class Host
       #
       # It stores sass custom functions and handles function calls.
       class FunctionRegistry
-        attr_reader :global_functions
+        attr_reader :environment, :global_functions
 
         def initialize(functions, alert_color:)
           functions = functions.transform_keys(&:to_s)
 
+          @environment = BasicObject.new
           @global_functions = functions.keys
           @functions_by_name = functions.transform_keys do |signature|
             index = signature.index('(')
diff --git a/lib/sass/compiler/host/protofier.rb b/lib/sass/compiler/host/protofier.rb
index 610e3f7..04497fc 100644
--- a/lib/sass/compiler/host/protofier.rb
+++ b/lib/sass/compiler/host/protofier.rb
@@ -64,6 +64,8 @@ def to_proto(obj)
             )
           when Sass::Value::Function
             if obj.instance_variable_defined?(:@id)
+              assert_compiler_value(obj)
+
               EmbeddedProtocol::Value.new(
                 compiler_function: EmbeddedProtocol::Value::CompilerFunction.new(
                   id: obj.instance_variable_get(:@id)
@@ -78,6 +80,8 @@ def to_proto(obj)
               )
             end
           when Sass::Value::Mixin
+            assert_compiler_value(obj)
+
             EmbeddedProtocol::Value.new(
               compiler_mixin: EmbeddedProtocol::Value::CompilerMixin.new(
                 id: obj.instance_variable_get(:@id)
@@ -148,17 +152,11 @@ def from_proto(proto)
               end
             )
           when :compiler_function
-            Sass::Value::Function.allocate.instance_eval do
-              @id = obj.id
-              self
-            end
+            compiler_value(Sass::Value::Function, obj.id)
           when :host_function
             raise Sass::ScriptError, 'The compiler may not send Value.host_function to host'
           when :compiler_mixin
-            Sass::Value::Mixin.allocate.instance_eval do
-              @id = obj.id
-              self
-            end
+            compiler_value(Sass::Value::Mixin, obj.id)
           when :calculation
             Calculation.from_proto(obj)
           when :singleton
@@ -177,6 +175,23 @@ def from_proto(proto)
           end
         end
 
+        private
+
+        def assert_compiler_value(value)
+          unless value.instance_variable_get(:@environment) == @function_registry.environment
+            raise Sass::ScriptError, "Returned #{value} does not belong to this compilation"
+          end
+
+          value
+        end
+
+        def compiler_value(klass, id)
+          value = klass.allocate
+          value.instance_variable_set(:@environment, @function_registry.environment)
+          value.instance_variable_set(:@id, id)
+          value
+        end
+
         # The {Number} Protofier.
         module Number
           module_function
diff --git a/lib/sass/value/function.rb b/lib/sass/value/function.rb
index eef9103..82a1046 100644
--- a/lib/sass/value/function.rb
+++ b/lib/sass/value/function.rb
@@ -15,6 +15,9 @@ def initialize(signature, &callback)
         @callback = callback.freeze
       end
 
+      # @return [Object, nil]
+      protected attr_reader :environment
+
       # @return [Integer, nil]
       protected attr_reader :id
 
@@ -26,10 +29,10 @@ def initialize(signature, &callback)
 
       # @return [::Boolean]
       def ==(other)
-        if id.nil?
-          other.equal?(self)
+        if defined?(@id)
+          other.is_a?(Sass::Value::Function) && other.environment == environment && other.id == id
         else
-          other.is_a?(Sass::Value::Function) && other.id == id
+          other.equal?(self)
         end
       end
 
diff --git a/lib/sass/value/mixin.rb b/lib/sass/value/mixin.rb
index 2f65fdb..469eb75 100644
--- a/lib/sass/value/mixin.rb
+++ b/lib/sass/value/mixin.rb
@@ -12,12 +12,15 @@ class << self
         private :new
       end
 
+      # @return [Object]
+      protected attr_reader :environment
+
       # @return [Integer]
       protected attr_reader :id
 
       # @return [::Boolean]
       def ==(other)
-        other.is_a?(Sass::Value::Mixin) && other.id == id
+        other.is_a?(Sass::Value::Mixin) && other.environment == environment && other.id == id
       end
 
       # @return [Integer]