Unable to send mail with attachment using Mandrill-api gem (Rails 4.1)

Emails without attachment are delivered fine, but when there is an attachment I get the following error in production:

ArgumentError: An SMTP To address is required to send a message. Set the message smtp_envelope_to, to, cc, or bcc address.

• Letter opener in dev model shows the email rendering perfectly
• This error only shows up in production

the call to mailer is:

# template variables
merge_vars = {"AMOUNT" => "100"}

invoice = Invoice.new(customer)

attachments[invoice.name] = {
  data: invoice.invoice_data,
  mime_type: 'application/pdf'
}

mail(to: user.email, cc: cc_emails.compact.uniq, subject: mail_subject, content_type: content_type) do |format|
  # Template renders fine
  format.html{mandrill_template('invoice_template', merge_vars)}
end

InvoiceMailer < ActionMailer::Base has defaults for "from" and "reply_to"

Mandril gem version 1.0.53 and Rails 4.1.10.

via Chebli Mohamed