diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 5c45e30950..33630463b2 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -180,6 +180,7 @@ check_modifiable(struct StringIO *ptr)
     else if (OBJ_FROZEN_RAW(ptr->string)) {
 	rb_raise(rb_eIOError, "not modifiable string");
     }
+    rb_str_modify(ptr->string);
 }
 
 static VALUE
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index 64bc5f67c3..8d236e5bba 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -14,6 +14,24 @@ class TestStringIO < Test::Unit::TestCase
 
   include TestEOF::Seek
 
+  def test_do_not_mutate_shared_buffers
+    # Ensure we have two strings that are not embedded but have the same shared
+    # string reference.
+    #
+    # In this case, we must use eval because we need two strings literals that
+    # are long enough they cannot be embedded, but also contain the same bytes.
+
+    a = eval(("x" * 1024).dump)
+    b = eval(("x" * 1024).dump)
+
+    s = StringIO.new(b)
+    s.getc
+    s.ungetc '#'
+
+    # We mutated b, so a should not be mutated
+    assert_equal("x", a[0])
+  end
+
   def test_version
     assert_kind_of(String, StringIO::VERSION)
   end