Notice: Undefined offset: 4 in wpmandrill.php on line 593

I am running into an error when using the wpMandrill::mail()/wp_mail() function.

Im submitting my forms with ajax and the response I get back is
Notice: Undefined offset: 4 in /wp-content/plugins/wpmandrill/wpmandrill.php on line 593

Im trying to run two of these functions at the same time. This is because I send a notification email to the admin and a thank you email to the user. 2 variants on email content and 2 different recipients.

I am using the function like so:
wp_mail($to_address, $subject_line, $body, $headers);

I have also tried wpMandrill::mail(); but still getting the same error.

I am receiving one of the emails but the other is not processing.

https://www.ads-software.com/plugins/wpmandrill/