4600 error on API updateRecords

4600 error on API updateRecords

I am testing the API trying to get some information from our on-premises database into Zoho CRM.

I have written a little c# app to do this. I am trying to update a single field, Test, which is a numeric field, within the Accounts module. My app produces the following query string. I have removed the auth token and record id from the string

https://crm.zoho.com/crm/private/xml/Accounts/updateRecords?authtoken=authtoken&scope=crmapi&id=recordid&XMLData=%3CAccounts%3E%3Crow%20no=%221%22%3E%3CFL%20val=%22Test%22%3E%3C![CDATA[100]]%3E%3C/FL%3E%3C/row%3E%3C/Accounts%3E

This is my XML

<Accounts>
<row no="1">
<FL val="Test">100</FL>
</row>
</Accounts>

This gives me  4600 error  - Unable to process your request. Please verify if the name and value is appropriate for the "xmlData" parameter.

Can anyone help?