API GetTicket - error if private note is added

[dotcomUNDERGROUND]

If a private note is added to a ticket, GetTicket api call is throwing error:

Oops!
Something went wrong and we couldn't process your request.

Error: Invalid string offset in /home/whmcs_dir/includes/api/getticket.php:0
Stack trace:
#0 /home/whmcs_dir/vendor/whmcs/whmcs-foundation/lib/Api/V1/Api.php(0): unknown()
#1 /home/whmcs_dir/vendor/whmcs/whmcs-foundation/lib/Api/V1/Api.php(0): WHMCS\Api\V1\Api->executeApiCall()
#2 /home/whmcs_dir/vendor/whmcs/whmcs-foundation/lib/Api/ApplicationSupport/Route/Middleware/HandleProcessor.php(0): WHMCS\Api\V1\Api->call()
#3 /home/whmcs_dir/vendor/whmcs/whmcs-foundation/lib/Api/ApplicationSupport/Route/Middleware/HandleProcessor.php(0): WHMCS\Api\ApplicationSupport\Route\Middleware\HandleProcessor->processV1Request(Object(WHMCS\Api\ApplicationSupport\Http\ServerRequest))
#4 /home/whmcs_dir/vendor/whmcs/whmcs-foundation/lib/Route/Middleware/Strategy/DelegatingMiddlewareTrait.php(0): WHMCS\Api\ApplicationSupport\Route\Middleware\HandleProcessor->_process(Object(WHMCS\Api\ApplicationSupport\Http\ServerRequest), Object(Middlewares\Utils\Delegate))
.
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If I remove the private note from the ticket, everything works fine.

Parameters I am passing:
 

$command = array(
'action' => 'GetTicket',
'username' => $whmcs_user,
'password' => $whmcs_pass,
'ticketnum' => $ticket_id,
'responsetype' => 'json',
);