diff --git a/lib/sassc/engine.rb b/lib/sassc/engine.rb
index 935b547d..a9d5449b 100644
--- a/lib/sassc/engine.rb
+++ b/lib/sassc/engine.rb
@@ -16,6 +16,7 @@ class Engine
     def initialize(template, options = {})
       @template = template
       @options = options
+      @functions = options.fetch(:functions, Script::Functions)
     end
 
     def render
@@ -37,7 +38,7 @@ def render
       Native.option_set_omit_source_map_url(native_options, true) if omit_source_map_url?
 
       import_handler.setup(native_options)
-      functions_handler.setup(native_options)
+      functions_handler.setup(native_options, functions: @functions)
 
       status = Native.compile_data_context(data_context)
 
diff --git a/lib/sassc/functions_handler.rb b/lib/sassc/functions_handler.rb
index 97a10973..8a3c3495 100644
--- a/lib/sassc/functions_handler.rb
+++ b/lib/sassc/functions_handler.rb
@@ -6,24 +6,24 @@ def initialize(options)
       @options = options
     end
 
-    def setup(native_options)
+    def setup(native_options, functions: Script::Functions)
       @callbacks = {}
       @function_names = {}
 
-      list = Native.make_function_list(Script.custom_functions.count)
+      list = Native.make_function_list(Script.custom_functions(functions: functions).count)
 
       # use an anonymous class wrapper to avoid mutations in a threaded environment
-      functions = Class.new do
+      functions_wrapper = Class.new do
         attr_accessor :options
-        include Script::Functions
+        include functions
       end.new
-      functions.options = @options
+      functions_wrapper.options = @options
 
-      Script.custom_functions.each_with_index do |custom_function, i|
+      Script.custom_functions(functions: functions).each_with_index do |custom_function, i|
         @callbacks[custom_function] = FFI::Function.new(:pointer, [:pointer, :pointer]) do |native_argument_list, cookie|
           begin
             function_arguments = arguments_from_native_list(native_argument_list)
-            result = functions.send(custom_function, *function_arguments)
+            result = functions_wrapper.send(custom_function, *function_arguments)
             to_native_value(result)
           rescue StandardError => exception
             # This rescues any exceptions that occur either in value conversion
@@ -32,7 +32,7 @@ def setup(native_options)
           end
         end
 
-        @function_names[custom_function] = Script.formatted_function_name(custom_function)
+        @function_names[custom_function] = Script.formatted_function_name(custom_function, functions: functions)
 
         callback = Native.make_function(
           @function_names[custom_function],
diff --git a/lib/sassc/script.rb b/lib/sassc/script.rb
index aecb9874..0eed756a 100644
--- a/lib/sassc/script.rb
+++ b/lib/sassc/script.rb
@@ -3,12 +3,12 @@
 module SassC
   module Script
 
-    def self.custom_functions
-      Functions.public_instance_methods
+    def self.custom_functions(functions: Functions)
+      functions.public_instance_methods
     end
 
-    def self.formatted_function_name(function_name)
-      params = Functions.instance_method(function_name).parameters
+    def self.formatted_function_name(function_name, functions: Functions)
+      params = functions.instance_method(function_name).parameters
       params = params.map { |param_type, name| "$#{name}#{': null' if param_type == :opt}" }.join(", ")
       return "#{function_name}(#{params})"
     end
diff --git a/test/functions_test.rb b/test/functions_test.rb
index 5cf7aeca..4b25c4bd 100644
--- a/test/functions_test.rb
+++ b/test/functions_test.rb
@@ -201,6 +201,17 @@ def test_concurrency
       end
     end
 
+    def test_pass_custom_functions_as_a_parameter
+      out = Engine.new("div { url: test-function(); }", {functions: ExternalFunctions}).render
+      assert_match /custom_function/, out
+    end
+
+    def test_pass_incompatible_type_to_custom_functions
+      assert_raises(TypeError) do
+        Engine.new("div { url: test-function(); }", {functions: Class.new}).render
+      end
+    end
+
     private
 
     def assert_sass(sass, expected_css)
@@ -319,5 +330,11 @@ def returns_sass_list
 
     end
 
+    module ExternalFunctions
+      def test_function
+        SassC::Script::Value::String.new("custom_function", :string)
+      end
+    end
+
   end
 end