diff --git a/lib/timeout.rb b/lib/timeout.rb
index eb66e58..903a831 100644
--- a/lib/timeout.rb
+++ b/lib/timeout.rb
@@ -282,19 +282,21 @@ def self.timeout(sec, klass = nil, message = nil, &block)   #:yield: +sec+
     message ||= "execution expired"
 
     if Fiber.respond_to?(:current_scheduler) && (scheduler = Fiber.current_scheduler)&.respond_to?(:timeout_after)
-      return scheduler.timeout_after(sec, klass || Error, message, &block)
-    end
-
-    state = State.instance
-    state.ensure_timeout_thread_created
-
-    perform = Proc.new do |exc|
-      request = Request.new(Thread.current, sec, exc, message)
-      state.add_request(request)
-      begin
-        return yield(sec)
-      ensure
-        request.finished
+      perform = Proc.new do |exc|
+        scheduler.timeout_after(sec, exc, message, &block)
+      end
+    else
+      state = State.instance
+      state.ensure_timeout_thread_created
+
+      perform = Proc.new do |exc|
+        request = Request.new(Thread.current, sec, exc, message)
+        state.add_request(request)
+        begin
+          return yield(sec)
+        ensure
+          request.finished
+        end
       end
     end
 
diff --git a/test/test_timeout.rb b/test/test_timeout.rb
index b11fc92..5db355a 100644
--- a/test/test_timeout.rb
+++ b/test/test_timeout.rb
@@ -459,4 +459,77 @@ def test_timeout_in_trap_handler
       trap(signal, original_handler)
     end
   end
+
+  if Fiber.respond_to?(:current_scheduler)
+    # Stubs Fiber.current_scheduler for the duration of the block, then restores it.
+    def with_mock_scheduler(mock)
+      original = Fiber.method(:current_scheduler)
+      Fiber.define_singleton_method(:current_scheduler) { mock }
+      begin
+        yield
+      ensure
+        Fiber.define_singleton_method(:current_scheduler, original)
+      end
+    end
+
+    def test_fiber_scheduler_delegates_to_timeout_after
+      received = nil
+      mock = Object.new
+      mock.define_singleton_method(:timeout_after) do |sec, exc, msg, &blk|
+        received = [sec, exc, msg]
+        blk.call(sec)
+      end
+
+      with_mock_scheduler(mock) do
+        assert_equal :ok, Timeout.timeout(5) { :ok }
+      end
+
+      assert_equal 5, received[0]
+      assert_instance_of Timeout::ExitException, received[1], "scheduler should receive an ExitException instance when no klass given"
+      assert_equal "execution expired", received[2]
+    end
+
+    def test_fiber_scheduler_delegates_to_timeout_after_with_custom_exception
+      custom_error = Class.new(StandardError)
+      received = nil
+      mock = Object.new
+      mock.define_singleton_method(:timeout_after) do |sec, exc, msg, &blk|
+        received = [sec, exc, msg]
+        blk.call(sec)
+      end
+
+      with_mock_scheduler(mock) do
+        assert_equal :ok, Timeout.timeout(5, custom_error, "custom message") { :ok }
+      end
+
+      assert_equal [5, custom_error, "custom message"], received
+    end
+
+    def test_fiber_scheduler_timeout_raises_timeout_error
+      mock = Object.new
+      mock.define_singleton_method(:timeout_after) do |sec, exc, msg, &blk|
+        raise exc  # simulate timeout firing
+      end
+
+      with_mock_scheduler(mock) do
+        assert_raise(Timeout::Error) do
+          Timeout.timeout(5) { :should_not_reach }
+        end
+      end
+    end
+
+    def test_fiber_scheduler_timeout_raises_custom_error
+      custom_error = Class.new(StandardError)
+      mock = Object.new
+      mock.define_singleton_method(:timeout_after) do |sec, exc, msg, &blk|
+        raise exc, msg
+      end
+
+      with_mock_scheduler(mock) do
+        assert_raise_with_message(custom_error, "custom message") do
+          Timeout.timeout(5, custom_error, "custom message") { :should_not_reach }
+        end
+      end
+    end
+  end
 end