[Puppet Users] Sometimes getting 'undef' in template

Morning, all. I've got a problem with a custom class and template that has me stumped. I've created the following class:

class graphite::carbon(
$cache_port = 2003,
$cache_enable_udp = false,
$cache_udp_port = $cache_port,
) {
package {'carbon': }

file {'/etc/carbon/carbon.conf':
content => template("graphite/carbon.conf.erb"),

require => Package['carbon'],
notify => Service['carbon-cache'],
}

service {'carbon-cache':
enable => true,
ensure => running,

require => Package['carbon'],
}
}

carbon.conf.erb contains this:

UDP_RECEIVER_PORT = <%= cache_udp_port %>

And I use the class like this:

class {'graphite::carbon': }

The problem I'm having is that, without making any changes *AT ALL* UDP_RECEIVER_PORT will sometimes have the default port of "2003" and other times "undef". This is with Puppet 2.7.17 in standalone mode.

Am I doing something wrong with the definition of cache_udp_port? I want it to default to the value provided for cache_port, which defaults to 2003.

Thanks,
Brian