diff --git a/lib/sassc/functions_handler.rb b/lib/sassc/functions_handler.rb
index c2237794..97a10973 100644
--- a/lib/sassc/functions_handler.rb
+++ b/lib/sassc/functions_handler.rb
@@ -12,7 +12,11 @@ def setup(native_options)
 
       list = Native.make_function_list(Script.custom_functions.count)
 
-      functions = FunctionWrapper.extend(Script::Functions)
+      # use an anonymous class wrapper to avoid mutations in a threaded environment
+      functions = Class.new do
+        attr_accessor :options
+        include Script::Functions
+      end.new
       functions.options = @options
 
       Script.custom_functions.each_with_index do |custom_function, i|
@@ -65,11 +69,5 @@ def error(message)
       $stderr.puts "[SassC::FunctionsHandler] #{message}"
       Native.make_error(message)
     end
-
-    class FunctionWrapper
-      class << self
-        attr_accessor :options
-      end
-    end
   end
 end
diff --git a/test/functions_test.rb b/test/functions_test.rb
index 71fc0fa8..5cf7aeca 100644
--- a/test/functions_test.rb
+++ b/test/functions_test.rb
@@ -185,6 +185,22 @@ def test_function_that_takes_a_sass_list
       CSS
     end
 
+    def test_concurrency
+      10.times do
+        threads = []
+        10.times do |i|
+          threads << Thread.new(i) do |id|
+            out = Engine.new("div { url: inspect_options(); }", {test_key1: 'test_value', test_key2: id}).render
+            assert_match /test_key1/, out
+            assert_match /test_key2/, out
+            assert_match /test_value/, out
+            assert_match /#{id}/, out
+          end
+        end
+        threads.each(&:join)
+      end
+    end
+
     private
 
     def assert_sass(sass, expected_css)
@@ -279,6 +295,10 @@ def inspect_list(argument)
         return argument
       end
 
+      def inspect_options
+        SassC::Script::Value::String.new(self.options.inspect, :string)
+      end
+
       def returns_sass_value
         return SassC::Script::Value::Color.new(red: 0, green: 0, blue: 0)
       end