1 files changed, 5 insertions, 4 deletions
diff --git a/string.c b/string.c
index f770118660..29dfdb94af 100644
--- a/string.c
+++ b/string.c
@@ -4567,13 +4567,14 @@ rb_str_setbyte(VALUE str, VALUE index, VALUE value)
 {
     long pos = NUM2LONG(index);
     int byte = NUM2INT(value);
+    long len = RSTRING_LEN(str);
 
-    rb_str_modify(str);
-
-    if (pos < -RSTRING_LEN(str) || RSTRING_LEN(str) <= pos)
+    if (pos < -len || len <= pos)
         rb_raise(rb_eIndexError, "index %ld out of string", pos);
     if (pos < 0)
-        pos += RSTRING_LEN(str);
+        pos += len;
+
+    rb_str_modify(str);
 
     RSTRING_PTR(str)[pos] = byte;