handle_bbcode_()

[MTGDarkness]

This feature was (I'm pretty sure) in vBulletin 3.6, but now it's giving me an error in 3.7.
I'm guessing that this line is what's causing the issue: 'external_callback' => 'handle_bbcode_x'

What it gives me is
Fatal error: Call to undefined function: handle_bbcode_x() in <snip>/forums/includes/class_bbcode.php on line 1317

Is there any way to work around this?

[Dismounted]

handle_bbcode_x() must be defined for it to work. The error says that the function is not defined.