diff --git a/lib/orthoses/content.rb b/lib/orthoses/content.rb
index 3e29e5a..85d5a52 100644
--- a/lib/orthoses/content.rb
+++ b/lib/orthoses/content.rb
@@ -68,7 +68,7 @@ def sort!
       require 'rbs/sorter'
       sorter = ::RBS::Sorter.new(nil, stdout: nil)
       decl = to_decl
-      sorter.sort_decl!(decl)
+      decl = sorter.sort_decl(decl)
       lines = decl_to_lines(decl)
       @body.replace(lines)
     end
diff --git a/lib/orthoses/sort_test.rb b/lib/orthoses/sort_test.rb
new file mode 100644
index 0000000..0bc1c1e
--- /dev/null
+++ b/lib/orthoses/sort_test.rb
@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+begin
+  require 'test_helper'
+rescue LoadError
+end
+
+module SortTest
+  def test_sort(t)
+    store = Orthoses::Builder.new do
+      use Orthoses::Sort
+      use Orthoses::Tap do |store|
+        store["Foo"].header = "class Foo"
+        store["Foo"] << "def foo: () -> void"
+        store["Foo"] << "def baz: () -> void"
+        store["Foo"] << "def bar: () -> void"
+      end
+      run -> () {}
+    end.call
+
+    actual = store.map { |n, c| c.to_rbs }.join("\n")
+    expect = <<~RBS
+      class Foo
+        def bar: () -> void
+        def baz: () -> void
+        def foo: () -> void
+      end
+    RBS
+    unless expect == actual
+      t.error("expect=\n```rbs\n#{expect}```\n, but got \n```rbs\n#{actual}```\n")
+    end
+  end
+end