[wp-testers] add_theme_support error?

[Jennifer Stuart]

I was going to test out post_thumbnails, added this to my config file:

add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 150, 150, true );

and got this error:

Fatal error: Call to undefined function add_theme_support()

(I'm new to the wp-testers list - so if there's a protocol for asking about issues - please let me know.)

-Jenn