Fatal error: Call to a member function add_cap()

Here's the "full" error (minus my server's directory structure):

Fatal error: Call to a member function add_cap() on a non-object in /wp-content/plugins/custompress/core/content-types.php on line 357

I migrated this site from one URL to another. I deleted the .htaccess file from the site root (because it was forwarding to the old site) and now when I try to load the site I get this error.