diff --git a/Rakefile b/Rakefile
index 6a6b7700..13e97e8a 100644
--- a/Rakefile
+++ b/Rakefile
@@ -4,22 +4,12 @@ rescue LoadError
   puts 'Cannot load bundler/gem_tasks'
 end
 
-task default: :prepare
+require 'tasks/libsass'
 
-task prepare: "ext/lib/libsass.so"
+task default: :test
 
-file "ext/lib/libsass.so" do
-  gem_dir = File.expand_path(File.dirname(__FILE__)) + "/"
-  cd "ext/libsass"
-  sh 'make lib/libsass.so LDFLAGS="-Wall -O2"'
-  cd gem_dir
-end
-
-task test: :prepare do
+desc "Run all tests"
+task test: 'libsass:compile' do
   $LOAD_PATH.unshift('lib', 'test')
   Dir.glob('./test/**/*_test.rb') { |f| require f }
 end
-
-task :submodule do
-  sh "git submodule update --init"
-end
diff --git a/ext/Rakefile b/ext/Rakefile
new file mode 100644
index 00000000..a8ed3958
--- /dev/null
+++ b/ext/Rakefile
@@ -0,0 +1,3 @@
+require_relative '../lib/tasks/libsass'
+
+task default: 'libsass:compile'
diff --git a/lib/sassc/functions_handler.rb b/lib/sassc/functions_handler.rb
index 3dc49b0c..b6c3c7d5 100644
--- a/lib/sassc/functions_handler.rb
+++ b/lib/sassc/functions_handler.rb
@@ -10,25 +10,45 @@ def setup(native_options)
 
       list = Native.make_function_list(Script.custom_functions.count)
 
-      functs = FunctionWrapper.extend(Script::Functions)
-      functs.options = @options
+      functions = FunctionWrapper.extend(Script::Functions)
+      functions.options = @options
 
       Script.custom_functions.each_with_index do |custom_function, i|
-        @callbacks[custom_function] = FFI::Function.new(:pointer, [:pointer, :pointer]) do |s_args, cookie|
-          length = Native.list_get_length(s_args)
+        @callbacks[custom_function] = FFI::Function.new(:pointer, [:pointer, :pointer]) do |native_argument_list, cookie|
+          native_argument_list_length = Native.list_get_length(native_argument_list)
+          custom_function_arguments = []
+          error_tag = nil
 
-          v = Native.list_get_value(s_args, 0)
-          v = Native.string_get_value(v).dup
+          (0...native_argument_list_length).each do |i|
+            native_value = Native.list_get_value(native_argument_list, i)
 
-          s = Script::String.new(Script::String.unquote(v), Script::String.type(v))
+            case value_tag = Native.value_get_tag(native_value)
+            when :sass_null
+              # no-op
+            when :sass_string
+              native_string = Native.string_get_value(native_value)
+              argument = Script::String.new(Script::String.unquote(native_string), Script::String.type(native_string))
 
-          value = functs.send(custom_function, s)
+              custom_function_arguments << argument
+            else
+              error_tag = error("Sass argument of type #{value_tag} unsupported")
+              break
+            end
+          end
+
+          next error_tag if error_tag
 
-          if value
-            value = Script::String.new(Script::String.unquote(value.to_s), value.type)
-            value.to_native
-          else
-            Script::String.new("").to_native
+          begin
+            value = functions.send(custom_function, *custom_function_arguments)
+
+            if value
+              value = Script::String.new(Script::String.unquote(value.to_s), value.type)
+              value.to_native
+            else
+              Script::String.new("").to_native
+            end
+          rescue StandardError => exception
+            error(exception.message)
           end
         end
 
@@ -48,6 +68,21 @@ def setup(native_options)
 
     private
 
+    def error(message)
+      value = Native::SassValue.new
+      value[:unknown] = Native::SassUnknown.new
+
+      error = Native::SassError.new
+      error[:tag] = :sass_error
+
+      Native.error_set_message(error, Native.native_string(message))
+      $stderr.puts "[SassC::FunctionsHandler] #{message}"
+
+      value[:unknown][:tag] = :sass_error
+      value[:error] = error
+      value.pointer
+    end
+
     class FunctionWrapper
       class << self
         attr_accessor :options
diff --git a/lib/sassc/native.rb b/lib/sassc/native.rb
index 469d1d99..dde78a86 100644
--- a/lib/sassc/native.rb
+++ b/lib/sassc/native.rb
@@ -48,7 +48,8 @@ def self.return_string_array(ptr)
     end
 
     def self.native_string(string)
-      string += "\0"
+      string = string.to_s
+      string << "\0"
       data = Native::LibC.malloc(string.bytesize)
       data.write_string(string)
       data
diff --git a/lib/sassc/native/native_functions_api.rb b/lib/sassc/native/native_functions_api.rb
index 8a7f5096..83678516 100644
--- a/lib/sassc/native/native_functions_api.rb
+++ b/lib/sassc/native/native_functions_api.rb
@@ -19,6 +19,7 @@ module Native
 
     # ADDAPI enum Sass_Tag ADDCALL sass_value_get_tag (const union Sass_Value* v);
     attach_function :sass_value_get_tag, [:sass_value_ptr], SassTag
+    attach_function :sass_value_is_null, [:sass_value_ptr], :bool
 
     # ADDAPI const char* ADDCALL sass_string_get_value (const union Sass_Value* v);
     attach_function :sass_string_get_value, [:sass_value_ptr], :string
@@ -28,6 +29,11 @@ module Native
     attach_function :sass_list_get_length, [:sass_value_ptr], :size_t
     attach_function :sass_list_get_value, [:sass_value_ptr, :size_t], :sass_value_ptr
 
+    # ADDAPI char* ADDCALL sass_error_get_message (const union Sass_Value* v);
+    # ADDAPI void ADDCALL sass_error_set_message (union Sass_Value* v, char* msg);
+    attach_function :sass_error_get_message, [:sass_value_ptr], :string
+    attach_function :sass_error_set_message, [:sass_value_ptr, :pointer], :void
+
     # Getters for custom function descriptors
     # ADDAPI const char* ADDCALL sass_function_get_signature (Sass_C_Function_Callback fn);
     # ADDAPI Sass_C_Function ADDCALL sass_function_get_function (Sass_C_Function_Callback fn);
diff --git a/lib/sassc/native/sass_value.rb b/lib/sassc/native/sass_value.rb
index f8950482..6da1003d 100644
--- a/lib/sassc/native/sass_value.rb
+++ b/lib/sassc/native/sass_value.rb
@@ -71,12 +71,12 @@ class SassNull < FFI::Struct
 
     class SassError < FFI::Struct
       layout :tag, SassTag,
-             :messsage, :string
+             :message, :string
     end
 
     class SassWarning < FFI::Struct
       layout :tag, SassTag,
-             :messsage, :string
+             :message, :string
     end
 
     class SassValue # < FFI::Union
diff --git a/lib/sassc/script.rb b/lib/sassc/script.rb
index f9b4f7d2..9704407f 100644
--- a/lib/sassc/script.rb
+++ b/lib/sassc/script.rb
@@ -8,10 +8,9 @@ def self.custom_functions
 
     def self.formatted_function_name(function_name)
       params = Functions.instance_method(function_name).parameters
-      params = params.select { |param| param[0] == :req }
-                     .map(&:first)
-                     .map { |p| "$#{p}" }
+      params = params.map { |param_type, name| "$#{name}#{': null' if param_type == :opt}" }
                      .join(", ")
+
       "#{function_name}(#{params})"
     end
   end
diff --git a/lib/sassc/script/string.rb b/lib/sassc/script/string.rb
index b41aa168..35050941 100644
--- a/lib/sassc/script/string.rb
+++ b/lib/sassc/script/string.rb
@@ -66,12 +66,12 @@ def self.type(contents)
       def self.unquote(contents)
         s = contents.dup
 
-        case contents[0,1]
+        case contents[0, 1]
         when "'", '"', '`'
           s[0] = ''
         end
 
-        case contents[-1,1]
+        case contents[-1, 1]
         when "'", '"', '`'
           s[-1] = ''
         end
diff --git a/lib/tasks/libsass.rb b/lib/tasks/libsass.rb
new file mode 100644
index 00000000..a659cd07
--- /dev/null
+++ b/lib/tasks/libsass.rb
@@ -0,0 +1,21 @@
+namespace :libsass do
+  desc "Compile libsass"
+  task compile: "ext/libsass/lib/libsass.so"
+
+  file "ext/libsass/.git" do
+    sh "git submodule update --init"
+  end
+
+  file "ext/libsass/lib/libsass.so" => "ext/libsass/.git" do
+    libsass_path = ""
+    if Dir.pwd.end_with?('/ext')
+      libsass_path = "libsass"
+    else
+      libsass_path = "ext/libsass"
+    end
+
+    cd libsass_path do
+      sh 'make lib/libsass.so LDFLAGS="-Wall -O2"'
+    end
+  end
+end
diff --git a/sassc.gemspec b/sassc.gemspec
index c7242bf0..0c31ea5f 100644
--- a/sassc.gemspec
+++ b/sassc.gemspec
@@ -16,9 +16,10 @@ Gem::Specification.new do |spec|
   spec.files         = `git ls-files -z`.split("\x0")
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
-  spec.require_paths = ["lib", "ext"]
 
-  spec.extensions    = ["Rakefile"]
+  spec.require_paths = ["lib"]
+
+  spec.extensions    = ["ext/Rakefile"]
 
   spec.add_development_dependency "rake"
   spec.add_development_dependency "minitest", "~> 5.5.1"
diff --git a/test/functions_test.rb b/test/functions_test.rb
index 2ebd0b99..529239d5 100644
--- a/test/functions_test.rb
+++ b/test/functions_test.rb
@@ -1,34 +1,16 @@
 require_relative "test_helper"
+require "stringio"
 
 module SassC
   class FunctionsTest < MiniTest::Test
     include FixtureHelper
 
-    SassString = Struct.new(:value, :type) do
-      def to_s
-        value
-      end
+    def setup
+      @real_stderr, $stderr = $stderr, StringIO.new
     end
 
-    module Script::Functions
-      def javascript_path(path)
-        Script::String.new("/js/#{path.value}", :string)
-      end
-
-      def no_return_path(path)
-        nil
-      end
-
-      def sass_return_path(path)
-        return SassString.new("'#{path.value}'", :string)
-      end
-
-      module Compass
-        def stylesheet_path(path)
-          Script::String.new("/css/#{path.value}", :identifier)
-        end
-      end
-      include Compass
+    def teardown
+      $stderr = @real_stderr
     end
 
     def test_functions_may_return_sass_string_type
@@ -70,5 +52,89 @@ def test_function_with_no_return_value
   url: url(); }
       EOS
     end
+
+    def test_function_with_optional_arguments
+      engine = Engine.new("div {url: optional_arguments('first'); url: optional_arguments('second', 'qux')}")
+      assert_equal <<-EOS, engine.render
+div {
+  url: "first/bar";
+  url: "second/qux"; }
+      EOS
+    end
+
+    def test_function_with_unsupported_tag
+      engine = Engine.new("div {url: function_with_unsupported_tag(red);}")
+
+      exception = assert_raises(SassC::SyntaxError) do
+        engine.render
+      end
+
+      assert_equal "Error: error in C function function_with_unsupported_tag: Sass argument of type sass_color unsupported\n\n       Backtrace:\n       \tstdin:1, in function `function_with_unsupported_tag`\n       \tstdin:1\n        on line 1 of stdin\n>> div {url: function_with_unsupported_tag(red);}\n   ----------^\n", exception.message
+
+      assert_equal "[SassC::FunctionsHandler] Sass argument of type sass_color unsupported", stderr_output
+    end
+
+    def test_function_with_error
+      engine = Engine.new("div {url: function_that_raises_errors();}")
+      exception = assert_raises(SassC::SyntaxError) do
+        engine.render
+      end
+
+      assert_equal "Error: error in C function function_that_raises_errors: Intentional wrong thing happened somewhere inside the custom function
+
+       Backtrace:
+       \tstdin:1, in function `function_that_raises_errors`
+       \tstdin:1
+        on line 1 of stdin
+>> div {url: function_that_raises_errors();}
+   ----------^
+", exception.message
+
+      assert_equal "[SassC::FunctionsHandler] Intentional wrong thing happened somewhere inside the custom function", stderr_output
+    end
+
+    private
+
+    SassString = Struct.new(:value, :type) do
+      def to_s
+        value
+      end
+    end
+
+    module Script::Functions
+      def javascript_path(path)
+        Script::String.new("/js/#{path.value}", :string)
+      end
+
+      def no_return_path(path)
+        nil
+      end
+
+      def sass_return_path(path)
+        return SassString.new("'#{path.value}'", :string)
+      end
+
+      def optional_arguments(path, optional = "bar")
+        return SassString.new("#{path}/#{optional}", :string)
+      end
+
+      def function_that_raises_errors()
+        raise StandardError, "Intentional wrong thing happened somewhere inside the custom function"
+      end
+
+      def function_with_unsupported_tag(color)
+      end
+
+      module Compass
+        def stylesheet_path(path)
+          Script::String.new("/css/#{path.value}", :identifier)
+        end
+      end
+      include Compass
+    end
+
+    def stderr_output
+      $stderr.string.gsub("\u0000\n", '')
+    end
   end
 end