diff --git a/string.c b/string.c
index e239a27a4c..4968e86d74 100644
--- a/string.c
+++ b/string.c
@@ -2386,7 +2386,7 @@ rb_str_subpos(VALUE str, long beg, long *lenp)
 	    beg += blen;
 	    if (beg < 0) return 0;
 	}
-	if (beg + len > blen)
+	if (len > blen - beg)
 	    len = blen - beg;
 	if (len < 0) return 0;
 	p = s + beg;