Cannot get custom objects from SOAP API

Hi,

I'm trying to get custom objects using the SOAP API but the call fails with this error:

<ns1:serviceException xmlns:ns1="http://www.marketo.com/mktows/"><name>mktValidationException</name><message>Unknown name key list: serviceName</message><code>10001</code></ns1:serviceException>

I have tested with several fields and double checked the spelling.

The code I'm using is this:

var parameters = new ParamsGetCustomObjects();
parameters.objTypeName = "<name of custom object, spelling is correct>";
parameters.batchSize = 100;
parameters.batchSizeSpecified = true;
parameters.customObjKeyList = new[]
{
new Attribute
{
attrName = "userEmail",
attrValue = request.UserEmail,
attrType = "string"
}
};

parameters.includeAttributes = new[]
{
"userEmail",
"serviceName"
};

var result = client.getCustomObjects(parameters);

I can for example get leads so the api and credentials are working.
What am I missing?

Thanks,
Rickard