Christiaan Conover on "get_plugin_data() is undefined function?"

I have the following code in my plugin's main file:

$_ENV['cc_author_plugindata'] = get_plugin_data( __FILE__, false );

I get the following error:

Fatal error: Call to undefined function get_plugin_data()

I'm inside wp-admin, so I'm not sure why this function isn't available. What's going wrong?