CiviMail Receipt Message contains "array" instead of variable value

I have an online contribution page that is used for a volunteer sign up. I've turned receipts on so that they can get a confirmation page with the events that they've signed up for.

I'm using version: 1.7.9821 Drupal PHP5

Would php-gd, or php-mbstring affect this?

The problem is that the content comes out like this:

Your request has been received, thank you for signing up to volunteer.

Please print this receipt for your records.
===========================================================
Membership Information
===========================================================
Membership Type: Volunteer
Membership Start Date: June 16th, 2007
Membership End Date:

===========================================================
Contribution Information
===========================================================
Amount: $ 0.00
Date: June 16th, 2007 9:52 PM

===========================================================
Name and Address
===========================================================
first_name : Array
last_name : Array
street_address-1 : Array
city-1 : Array
postal_code-1 : Array
state_province-1 : Array
phone-1-Phone : Array

===========================================================
Organization Volunteer
===========================================================
custom_7 : Array
custom_1 : Array
custom_2 : Array
custom_3 : Array
custom_4 : Array
custom_5 : Array
custom_6 : Array