Payment_total_type throwing json warning

We are not using payments in our form, but we’re running into a php notice that’s affecting our submission message:

Notice: Undefined index: payment_total_type in … plugins/ninja-forms/includes/Abstracts/MergeTags.php on line 73

The code in question checks to see if “payment_total” is a key in the $subject field, but not “payment_total_type”:

if (
                isset($subject['payment_total'])
                && $subject['payment_total_type'] == 'field'
                && ! is_numeric($subject['payment_total'])
                && ! strpos($subject['payment_total'], ':calc}')
            ) 

When I output the $subject array, there is no payment_total_type in the array, but there is an empty payment_total:

...
 [payment_gateways] => 
    [payment_total] => 
    [tag] => 
...

It appears that the isset($subject[“payment_total”]) is returning true, but since there’s not a key of “payment_total_type” in the $subject array, that is throwing the undefined index notice.