Warning: Illegal offset type in drupal_set_message()

Hi, I am not sure if this is actually an issue with certificate or print module. In an attempt to locate the exact issue, I created several clean test sites and tried both dev and supported versions of certificate and print.

Reports error:

Warning: Illegal offset type in drupal_set_message() (line 1763 of /home/quickstart/websites/certificate.dev/includes/bootstrap.inc).

Referrer: http://certificate.dev/

Steps I took: Clean drupal 7.19 site, Certificate 7.x-2.0-beta3, Print 7.x-2.0-beta2, put dompdf 0.6.0 beta 3 in libraries folder
1. Created test certificate
2. Was able to view certificate through the 'edit' and 'view' links
3. Was not able to view through pdf link, received Illegal offset error

I used the dev versions of certificate and print module as there were also issues with the supported versions. I would appreciate any help with this! Thanks