diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 975a308..e6b17a8 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -728,6 +728,7 @@ strio_ungetc(VALUE self, VALUE c)
     rb_encoding *enc, *enc2;
 
     if (NIL_P(c)) return Qnil;
+    check_modifiable(ptr);
     if (FIXNUM_P(c)) {
 	int cc = FIX2INT(c);
 	char buf[16];
@@ -792,6 +793,7 @@ strio_ungetbyte(VALUE self, VALUE c)
 	cl = RSTRING_LEN(c);
 	if (cl == 0) return Qnil;
     }
+    check_modifiable(ptr);
     rb_str_modify(str);
     if (cl > pos) {
 	char *s;
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index 417a53b..0ea3e4f 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -504,6 +504,14 @@ class TestStringIO < Test::Unit::TestCase
     assert_raise(RuntimeError, bug) {s.reopen("")}
   end
 
+  def test_frozen_string
+    s = StringIO.new("".freeze)
+    bug = '[ruby-core:48530]'
+    assert_raise(IOError, bug) {s.write("foo")}
+    assert_raise(IOError, bug) {s.ungetc("a")}
+    assert_raise(IOError, bug) {s.ungetbyte("a")}
+  end
+
   def test_readlines_limit_0
     assert_raise(ArgumentError, "[ruby-dev:43392]") { StringIO.new.readlines(0) }
   end