diff --git a/test/ruby/test_io_buffer.rb b/test/ruby/test_io_buffer.rb
index d5ba3266c7..1b4a09dd20 100644
--- a/test/ruby/test_io_buffer.rb
+++ b/test/ruby/test_io_buffer.rb
@@ -102,11 +102,6 @@ class TestIOBuffer < Test::Unit::TestCase
     IO::Buffer.for(string) do |buffer|
       refute buffer.readonly?
 
-      # Cannot modify string as it's locked by the buffer:
-      assert_raise RuntimeError do
-        string[0] = "h"
-      end
-
       buffer.set_value(:U8, 0, "h".ord)
 
       # Buffer releases it's ownership of the string:
@@ -116,6 +111,16 @@ class TestIOBuffer < Test::Unit::TestCase
     end
   end
 
+  def test_string_mapped_buffer_locked
+    string = "Hello World"
+    IO::Buffer.for(string) do |buffer|
+      # Cannot modify string as it's locked by the buffer:
+      assert_raise RuntimeError do
+        string[0] = "h"
+      end
+    end
+  end
+
   def test_non_string
     not_string = Object.new