diff --git a/string.c b/string.c
index 40b46eee57..a32b545642 100644
--- a/string.c
+++ b/string.c
@@ -2835,11 +2835,12 @@ rb_str_subpos(VALUE str, long beg, long *lenp)
 {
     long len = *lenp;
     long slen = -1L;
-    long blen = RSTRING_LEN(str);
+    const long blen = RSTRING_LEN(str);
     rb_encoding *enc = STR_ENC_GET(str);
     char *p, *s = RSTRING_PTR(str), *e = s + blen;
 
     if (len < 0) return 0;
+    if (beg < 0 && -beg < 0) return 0;
     if (!blen) {
         len = 0;
     }
@@ -2857,7 +2858,8 @@ rb_str_subpos(VALUE str, long beg, long *lenp)
     }
     if (beg < 0) {
         if (len > -beg) len = -beg;
-        if (-beg * rb_enc_mbmaxlen(enc) < RSTRING_LEN(str) / 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;
@@ -2875,7 +2877,7 @@ rb_str_subpos(VALUE str, long beg, long *lenp)
             if (len == 0) goto end;
         }
     }
-    else if (beg > 0 && beg > RSTRING_LEN(str)) {
+    else if (beg > 0 && beg > blen) {
         return 0;
     }
     if (len == 0) {
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index 24527a527f..f4ef6ccb6b 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -162,6 +162,15 @@ CODE
     assert_raise(ArgumentError) { "foo"[] }
   end
 
+  def test_AREF_underflow
+    require "rbconfig/sizeof"
+    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')
diff --git a/version.h b/version.h
index bfe737dfe5..30cc09bf05 100644
--- a/version.h
+++ b/version.h
@@ -11,7 +11,7 @@
 # define RUBY_VERSION_MINOR RUBY_API_VERSION_MINOR
 #define RUBY_VERSION_TEENY 6
 #define RUBY_RELEASE_DATE RUBY_RELEASE_YEAR_STR"-"RUBY_RELEASE_MONTH_STR"-"RUBY_RELEASE_DAY_STR
-#define RUBY_PATCHLEVEL 240
+#define RUBY_PATCHLEVEL 241
 
 #include "ruby/version.h"
 #include "ruby/internal/abi.h"