1 files changed, 4 insertions, 2 deletions
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);