_ pattern succeeds on end of input?

[pnkfelix@…]

The behavior below does not seem to agree with Bryan Ford's POPL '04 paper:

#lang scheme
(require (planet kazzmir/peg:2:0/peg))
(parse (peg (start s) (grammar (s ((_) $)))) "a")
(parse (peg (start s) (grammar (s ((_ _) $)))) "a")

produces the output:

Welcome to DrScheme, version 4.1.4 [3m].
Language: Module; memory limit: 128 megabytes.
#\a
#<procedure:end-of-input>
> 

I would have expected #f for that last output.