diff --git a/string.c b/string.c
index c9db5d35c5..b970c5c6f5 100644
--- a/string.c
+++ b/string.c
@@ -3076,7 +3076,8 @@ rb_str_subpos(VALUE str, long beg, long *lenp)
     }
     if (beg < 0) {
         if (len > -beg) len = -beg;
-        if (-beg * rb_enc_mbmaxlen(enc) < blen / 8) {
+        if ((ENC_CODERANGE(str) == ENC_CODERANGE_VALID) &&
+            (-beg * rb_enc_mbmaxlen(enc) < blen / 8)) {
             beg = -beg;
             while (beg-- > len && (e = rb_enc_prev_char(s, e, e, enc)) != 0);
             p = e;
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index c8b9aeb597..d2099607fd 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -169,6 +169,10 @@ CODE
     assert_equal(nil, S("\u{3042 3044 3046}")[RbConfig::LIMITS["LONG_MIN"], 1])
   end
 
+  def test_AREF_invalid_encoding
+    assert_equal(S("\x80"), S("A"*39+"\x80")[-1, 1])
+  end
+
   def test_ASET # '[]='
     s = S("FooBar")
     s[0] = S('A')