'InvoicePaid' Hook Not Firing?

[e-baker]

I am attempting to send invoice/client details to an external API every time an invoice is marked paid. I've created a simple script in an effort to capture the actual variables that are being passed.

 

Here's the problem: it appears the hook is not firing no matter the invoice amount.

 

add_hook('InvoicePaid', 5, function($vars) {
$output = "This is the output of the file: \n";
$output .= print_r($vars, true);
$file = fopen( "invoicepayment.log", "w" );
fwrite( $file, $output );
fclose( $file );});

 

I've tried searching but have had no success.