diff --git a/test/-ext-/string/test_external_new.rb b/test/-ext-/string/test_external_new.rb
index f7c7908db7..f8ee773b16 100644
--- a/test/-ext-/string/test_external_new.rb
+++ b/test/-ext-/string/test_external_new.rb
@@ -3,6 +3,12 @@ require 'test/unit'
 require '-test-/string'
 
 class Test_StringExternalNew < Test::Unit::TestCase
+  def test_buf_new
+    assert_operator(0, :<=, Bug::String.capacity(Bug::String.buf_new(0)))
+    assert_operator(127, :<=, Bug::String.capacity(Bug::String.buf_new(127)))
+    assert_operator(128, :<=, Bug::String.capacity(Bug::String.buf_new(128)))
+  end
+
   def test_external_new_with_enc
     Encoding.list.each do |enc|
       assert_equal(enc, Bug::String.external_new(0, enc).encoding)