Sagepay Simulator Error (CardHolder field) - php

Question

I’m trying to make a purchase request to a Simulator end point (https://test.sagepay.com/Simulator/VSPDirectGateway.asp), with the following data:
array(35) {
["VPSProtocol"]=>
string(4) "2.23"
["TxType"]=>
string(7) "PAYMENT"
["Vendor"]=>
string(9) "MY VENDOR NAME"
["AccountType"]=>
string(1) "E"
["Description"]=>
string(13) "My Test Order"
["Amount"]=>
string(4) "1.50"
["Currency"]=>
string(3) "GBP"
["VendorTxCode"]=>
string(1) "3"
["ClientIPAddress"]=>
NULL
["ApplyAVSCV2"]=>
int(0)
["Apply3DSecure"]=>
int(0)
["BillingFirstnames"]=>
string(3) "Sam"
["BillingSurname"]=>
string(4) "King"
["BillingAddress1"]=>
string(1) "1"
["BillingAddress2"]=>
string(1) "2"
["BillingCity"]=>
string(8) "Brighton"
["BillingPostCode"]=>
string(3) "BN1"
["BillingState"]=>
string(0) ""
["BillingCountry"]=>
string(2) "GB"
["BillingPhone"]=>
NULL
["DeliveryFirstnames"]=>
NULL
["DeliverySurname"]=>
NULL
["DeliveryAddress1"]=>
NULL
["DeliveryAddress2"]=>
NULL
["DeliveryCity"]=>
NULL
["DeliveryPostCode"]=>
NULL
["DeliveryState"]=>
string(0) ""
["DeliveryCountry"]=>
NULL
["DeliveryPhone"]=>
NULL
["CustomerEMail"]=>
NULL
["CardHolder"]=>
string(8) "Sam King"
["CardNumber"]=>
string(16) "4012888888881881"
["CV2"]=>
string(3) "123"
["ExpiryDate"]=>
string(4) "0215"
["CardType"]=>
string(4) "visa"
}
However, all I repeatedly keep getting back from Sagepay is a validation error:
"The CardHolder field should be between 1 and 50 characters long.”
However, as you can see in the above data array, I’m sending a CardHolder value of ‘Sam King’, which is clearly between 1 and 50 characters long.
For reference, I'm using the Omnipay (https://github.com/thephpleague/omnipay) library to make requests.
Any idea what I could be doing wrong here?