Ruby on Rails

How in the !@#$% can this function:

def sendMsg(msg, today, cancel)
 
email = <<MSG
Date: #{Time.now}
From: f...@bar.com
Subject: Pediatric Cardiology Apt#{cancel ? ' Cancellation' : ''}
Mime-Version: 1.0
Content-Type: text/html; charset="utf-8";
X-Priority: #{today ? '1' : '3'}

#{msg.encode('US-ASCII', {undef: :replace, invalid: :replace})}
MSG

  Net::SMTP.start("foo-...@bar.org", 25) do |smtp|
    smtp.send_mail(email, MAILUSER, ["some...@bar.org"])
  end

end

intermittently produce this error:

/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/net/protocol.rb:325:in `slice!': invalid byte sequence in US-ASCII (ArgumentError)

This is on OS X, and it only happens when run automatically from a launch daemon. Logging into the same account and running it manually always works. So it's some version or configuration issue. But I find that using rvm on OS X is frustrating to say the least...