Puppet users - Oct 2014 - How to ensure a variable gets set, before a File or Service gets set.

So I have the following puppet rule. Pasted below. The problem is, 
sometimes, I see it looks like it is running the File before it sets the 
variable $sendmail_file in the case statement above. The reason I believe 
this, is because of the error message that it reports. Which is

Error Message:
err: /File[sendmailcf]/ensure: change from absent to file failed: Could not 
set 'file on ensure: No such file or directory - 
/etc/mail/sendmail.cf.puppettmp_6850 at 
/etc/puppet/modules/sendmail/manifests/client.pp:40

What I can't figure out, is how to ensure that that variable gets set, 
before it gets used. Any pointers?

My puppet rule.
class sendmail::client {
  case $::operatingsystem {
    /^(RedHat|CentOS)/: {
      case $::lsbmajdistrelease  {
        '5': {
             $sendmail_file = 'sendmail.cf.rhel5'
        }
        '6': {
             $sendmail_file = 'sendmail.cf.rhel6'
        }
        default: {
          fail("Unsupported OS Major version ${::operatingsystem} 
${::lsbmajdistrelease}")
        }
      }
    }
    default: {
      fail("Unsupported operating system: ${::operatingsystem}")
    }
  }

  service { sendmail:
    name   => sendmail,
    enable => true,
    ensure => running,
  }

  file { sendmailcf:
    path => "/etc/mail/sendmail.cf",
    mode => 644,
    owner => root,
    group => root,
    ensure => present,
    source => "puppet:///modules/sendmail/$sendmail_file",
    notify => Service[sendmail],
  }
}

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