Echoing Text in Footer via Plugin

On my own blog, I am trying to put together a little plugin that would so some fancy dynamic stuff and then output the the resulting variable via the wp_footer() hook. Essentially, the plugin looks like this:

function get_my_custom_variable() {
	// *clip function contents*

	echo $resulting_variable;
}
// hook to WordPress footer
add_action( 'wp_footer', 'get_my_custom_variable', 10 );

What happens when I activate the plugin is that my blog ceases to load — no errors, no warnings, just a blank page w/ no outputted HTML.

If I comment out the last add_action() call and simply call the function itself, it works just fine when I access the plugin file directly via my browser. It seems to cause WordPress to choke, however.

Any tips on how I can debug this? It seems so simple!