diff --git a/string.c b/string.c
index 4968e86d74..43b8b4e007 100644
--- a/string.c
+++ b/string.c
@@ -5254,7 +5254,7 @@ str_byte_substr(VALUE str, long beg, long len, int empty)
 	beg += n;
 	if (beg < 0) return Qnil;
     }
-    if (beg + len > n)
+    if (len > n - beg)
 	len = n - beg;
     if (len <= 0) {
 	if (!empty) return Qnil;