Issue with the at matcher in PHPUnit

FooTest.php

<?php
class Bar {
    public function run() {
        // do something
    }

    public function exec() {
        // do something
    }
}

class Foo {
    private $bar;

    public function __construct(Bar $bar) {
        $this->bar = $bar;
    }

    public function runBar() {
        var_dump($this->bar->run());
        var_dump($this->bar->run());
        var_dump($this->bar->run());
    }

    public function execBar() {
        var_dump($this->bar->exec());
    }
}

class FooTest extends PHPUnit_Framework_TestCase {
    public function testFoo() {
        $bar = $this->getMock('Bar');
        $bar->expects($this->any())->method('exec')->will($this->returnValue('exec'));
        $bar->expects($this->at(0))->method('run')->will($this->returnValue('first run'));
        $bar->expects($this->at(1))->method('run')->will($this->returnValue('second run'));
        $bar->expects($this->at(2))->method('run')->will($this->returnValue('third run'));

        $foo = new Foo($bar);
        $foo->execBar();
        $foo->runBar();
    }
}

// Output:
// string(4) "exec"
// string(10) "second run"
// string(9) "third run"
// NULL