Can't add new task via API

Can't add new task via API

I'm trying to insert a new task and I can't seem to get it to work.

I'm doing a post to this url "https://crm.zoho.com/crm/private/xml/Tasks/insertRecords?authtoken=MYAPIKEY&scope=crmapi&newFormat=1" (using my real api key of course)

with these headers
User-Agent: Fiddler
Host: crm.zoho.com
Content-Length: 183
Content-Type: text/xml


with this request body

<Tasks>
<row no="1">
<FL val="Subject">Call</FL>
<FL val="Due Date">2009-04-20</FL>
<FL val="Status">Deferred</FL>
<FL val="CONTACTID">AVALIDID</FL> 
</row>
</Tasks>

with AVALIDID being some valid id that I use

and I keep getting a 
<response><error><code>4600</code><message>Unable to process your request. Please verify if the name and value is appropriate for the "xmlData" parameter.</message></error></response>

Can someone tell me what I'm doing wrong?