diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index ba6512366a..d6833976af 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -767,12 +767,14 @@ strio_ungetc(VALUE self, VALUE c)
     check_modifiable(ptr);
     if (NIL_P(c)) return Qnil;
     if (FIXNUM_P(c)) {
-	int cc = FIX2INT(c);
+	int len, cc = FIX2INT(c);
 	char buf[16];
 
 	enc = rb_enc_get(ptr->string);
+	len = rb_enc_codelen(cc, enc);
+	if (len <= 0) rb_enc_uint_chr(cc, enc);
 	rb_enc_mbcput(cc, buf, enc);
-	return strio_unget_bytes(ptr, buf, rb_enc_codelen(cc, enc));
+	return strio_unget_bytes(ptr, buf, len);
     }
     else {
 	SafeStringValue(c);
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index d4df7b2617..30370f5ca6 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -453,6 +453,8 @@ class TestStringIO < Test::Unit::TestCase
     f.ungetc("y".ord)
     assert_equal("y", f.getc)
     assert_equal("2", f.getc)
+
+    assert_raise(RangeError) {f.ungetc(0x1ffffff)}
   ensure
     f.close unless f.closed?
   end