Input string was not in a correct format

I'm getting an error when trying to post orders to Xero.

The error message, which I understand is fairly generic, is:

Error 14 PostDataInvalidException: Input string was not in a correct format

Here's a snippet from the error log that I'm getting. My initial thinking was that the problem lay with the TaxType being empty, but having looked at the code it seems that this is unset by the module so it can be handled by Xero. But I can also see that the LineAmount is also empty.

If it's relevant, the site is using the Multicurrency module, and posting to a multi-currency Xero account. The order below was made in NZD, which is the primary currency of the Xero account.

Thanks for any help you can provided in troubleshooting this.

Array
(
    [Invoice] => Array
        (
            [Type] => ACCREC
            [Contact] => Array
                (
                    [Name] => customername
                    [EmailAddress] => customer@example.com
                )

            [Date] => 2015-07-31
            [DueDate] => 2015-07-31
            [LineAmountTypes] => Inclusive
            [LineItems] => Array
                (
                    [0] => Array
                        (
                            [LineItem] => Array
                                (
                                    [Description] => UDT001
                                    [Quantity] => 1.00
                                    [UnitAmount] => 180.85
                                    [AccountCode] => 200
                                    [TaxType] => 
                                    [TaxAmount] => 0.00
                                    [LineAmount] => 
                                )

                        )

                    [1] => Array
                        (
                            [LineItem] => Array
                                (
                                    [Description] => Courier tracked 10-14 working days 
                                    [Quantity] => 1.00
                                    [UnitAmount] => 39.88
                                    [AccountCode] => 200
                                    [TaxType] => 
                                    [TaxAmount] => 0.00
                                    [LineAmount] => 
                                )

                        )

                    [2] => Array
                        (
                            [LineItem] => Array
                                (
                                    [Description] => 
                                    [Quantity] => 1.00
                                    [UnitAmount] => -20.00
                                    [AccountCode] => 200
                                    [TaxType] => 
                                    [TaxAmount] => 0.00
                                    [LineAmount] => 
                                )

                        )

                )

            [Reference] => Order #599
            [Status] => AUTHORISED
            [SubTotal] => 200.73
            [TotalTax] => 0.00
            [Total] => 222.88
            [Url] => https://www.example.com/admin/commerce/orders/599
        )

)