I keep getting a can't convert nil to String but can't debugy in Rails 3

How can I code this so that the exception will output which method and specific instance of contact is the problem?

 sub_message =
 13       message.gsub("{FirstName}", contact.first_name).
 14            gsub("{LastName}", contact.last_name).
 15            gsub("{Title}", contact.title || "blank").
 16    开发者_高级运维        gsub("{Signature}", contact.user.signature.to_s).
 17        #    gsub("{Company}", contact.company_name.clear_company).
 18            gsub("{Colleagues}", colleagues.to_sentence).
 19            gsub("{NextWeek}", next_week.strftime("%A, %B %d")).
 20            gsub("{FollowingWeek}", (Date.today + 14.days).strftime("%A, %B %d")).
 21            gsub("{UserFirstName}", contact.user.first_name).
 22            gsub("{UserLastName}", contact.user.last_name).
 23            gsub("{City}", contact.address.city.titleize || "default city").
 24            gsub("{State}", contact.address.state || "default state").
 25            gsub("{Zip}", contact.address.zip || "default zip" ).
 26            gsub("{Phone}", contact.phone.format_phone || "default phone" ).
 27            gsub("{Street1}", contact.address.street1.titleize || "default street").
 28            gsub("{Today}", (Date.today).strftime("%A, %B %d")).
 29            gsub("{CustomField1}", contact.custom_field_1.to_s || "custom").
 30            gsub("{PageBreak}", "p{page-break-after: always}. ")

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I would simply try a single message.gsub!() (mind the '!') per line, one for each attribute.

message.gsub!("{FirstName}", contact.first_name)
message.gsub!("{LastName}", contact.last_name)

This will save the message variable each time if something was changed, otherwise, it simply moves on to the next instance of gsub!.