Why am I getting a Call to a member function getEntitlements() on null error when I try to getEntitlements in php?

Question

     $marketplaceentitlementservice = new MarketplaceEntitlementServiceClient($config);
     $request = [];
     $request['ProductCode']=ProductCode;
     $filter = ['CUSTOMER_IDENTIFIER'=>[$result['CustomerIdentifier']]];
     $request['Filter']=$filter;
     $entitlement = $marketplaceentitlementservice->getEntitlements( $request);

I am trying to getEntitlements from AWS Marketplace SDK into $entitlement. I have setup the $request to be but I get that error.

Array
(
    [ProductCode] => 4gax08f1iim5hq4xcxxxw1akgvd
    [Filter] => Array
        (
            [CUSTOMER_IDENTIFIER] => Array
                (
                    [0] => 5JFYLyyynRmLY
                )

        )

)