diff --git a/string.c b/string.c
index b7ae7069f6..75db0d22e8 100644
--- a/string.c
+++ b/string.c
@@ -8026,7 +8026,8 @@ rb_str_enumerate_lines(int argc, VALUE *argv, VALUE str, VALUE ary)
 	    if (rsnewline) {
 		pend = chomp_newline(subptr, pend, enc);
 	    }
-	    else if (memcmp(pend - rslen, rsptr, rslen) == 0) {
+	    else if (pend - subptr >= rslen &&
+		     memcmp(pend - rslen, rsptr, rslen) == 0) {
 		pend -= rslen;
 	    }
 	}