2 files changed, 16 insertions, 2 deletions
diff --git a/io_buffer.c b/io_buffer.c
index 40c12ef5c1..190b67d8ac 100644
--- a/io_buffer.c
+++ b/io_buffer.c
@@ -496,7 +496,9 @@ io_buffer_for_yield_instance(VALUE _arguments)
 
     arguments->instance = io_buffer_for_make_instance(arguments->klass, arguments->string, arguments->flags);
 
-    rb_str_locktmp(arguments->string);
+    if (!RB_OBJ_FROZEN(arguments->string)) {
+        rb_str_locktmp(arguments->string);
+    }
 
     return rb_yield(arguments->instance);
 }
@@ -510,7 +512,9 @@ io_buffer_for_yield_instance_ensure(VALUE _arguments)
         rb_io_buffer_free(arguments->instance);
     }
 
-    rb_str_unlocktmp(arguments->string);
+    if (!RB_OBJ_FROZEN(arguments->string)) {
+        rb_str_unlocktmp(arguments->string);
+    }
 
     return Qnil;
 }
diff --git a/test/ruby/test_io_buffer.rb b/test/ruby/test_io_buffer.rb
index 55296c1f23..70c5ef061d 100644
--- a/test/ruby/test_io_buffer.rb
+++ b/test/ruby/test_io_buffer.rb
@@ -121,6 +121,16 @@ class TestIOBuffer < Test::Unit::TestCase
     end
   end
 
+  def test_string_mapped_buffer_frozen
+    string = "Hello World".freeze
+    IO::Buffer.for(string) do |buffer|
+      assert_raise IO::Buffer::AccessError, "Buffer is not writable!" do
+        buffer.set_string("abc")
+      end
+      assert_equal "H".ord, buffer.get_value(:U8, 0)
+    end
+  end
+
   def test_non_string
     not_string = Object.new