diff --git a/string.c b/string.c
index 43b8b4e007..be74fd8b72 100644
--- a/string.c
+++ b/string.c
@@ -4435,12 +4435,14 @@ rb_str_update(VALUE str, long beg, long len, VALUE val)
 	rb_raise(rb_eIndexError, "index %ld out of string", beg);
     }
     if (beg < 0) {
-	if (-beg > slen) {
+	if (beg + slen < 0) {
 	    goto out_of_range;
 	}
 	beg += slen;
     }
-    if (slen < len || slen < beg + len) {
+    assert(beg >= 0);
+    assert(beg <= slen);
+    if (len > slen - beg) {
 	len = slen - beg;
     }
     str_modify_keep_cr(str);
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index da8bd5f05a..39cfc9a220 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -202,6 +202,8 @@ CODE
     assert_equal("fobar", s)
 
     assert_raise(ArgumentError) { "foo"[1, 2, 3] = "" }
+
+    assert_raise(IndexError) {"foo"[RbConfig::Limits["LONG_MIN"]] = "l"}
   end
 
   def test_CMP # '<=>'