NoMethodError: undefined method 'index' when calling deliver on rails 3.1 mailer

Question

I have set up a staging instance of a rails 3.1 application on Heroku and everything seems to work fine except I'm getting a strange error when I try to send emails. I am using the sendgrid starter addon for email delivery. The full error is below:

NoMethodError: undefined method `index' for #<Mail::Message:0x000000048daf28>
    /app/.bundle/gems/ruby/1.9.1/gems/mail-2.3.0/lib/mail/message.rb:1289:in `method_missing'
    /app/.bundle/gems/ruby/1.9.1/gems/mail-2.3.0/lib/mail/encodings.rb:117:in `value_decode'
    /app/.bundle/gems/ruby/1.9.1/gems/mail-2.3.0/lib/mail/encodings.rb:101:in `decode_encode'
    /app/.bundle/gems/ruby/1.9.1/gems/mail-2.3.0/lib/mail/fields/unstructured_field.rb:74:in `do_decode'

if I just generate the message object without calling deliver on it and inspect it everything seems fine. I am not seeing this error on my production app. Can you tell me what this error means and how to resolve it? Thanks.

Answer 1

Got the same error on my local machine using Rails 3.0.11. It happened after I passed some object instead of a string to the mail :to attribute. So be sure that the :to attribute is a string!

mail(to: object.to_s)

Answer 2

A new version of Mail, Mail 2.4.0, was released this weekend. I would recommend upgrading to this latest version and seeing if that has fixed your issue.