Error message attempting to add a poller

I'm attempting to add a response-time poller to my recently created node, and getting an error message I don't understand.  it tells me that 'properties' must not be null.  Here is the soap message sent (edited for readability, and to remove all the excess namespace declarations...)

<?xml version="1.0" encoding="UTF-8"?>

<soap:Envelope>

<soap:Body>

<tns:Create>

<entityType xmlns="http://schemas.solarwinds.com/2007/08/informationservice" xsi:type="xsd:string">Orion.Pollers</entityType>

<properties>

<item>

<key>PollerType</key>

<type>System.String</type>

<value>Poller RT</value>

</item>

<item>

<key>NetObject</key>

<type>System.String</type>

<value>N:52</value>

</item>

</properties>

</tns:Create>

</soap:Body>

</soap:Envelope>