AS3 E4X, custom namespaces but no default namespace

I've seen plenty of posts about getting namespaced xml data, but this is making me bonkers. I am hitting a web service that is responding with some xml:

<?xml version="1.0" encoding="utf-8"?>
<response xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY">
  <host_name>hostname</host_name>
  <keyField>
    <fieldType>1</fieldType>
    <fieldTypeKey>job_id</fieldTypeKey>
    <stringValue>38252BCD-9FEF-4904-9AC0-8BF88F8B35B0</stringValue>
  </keyField>
  <field>
    <fieldType>1</fieldType>
    <fieldTypeKey>alarm</fieldTypeKey>
    <stringValue>channel 1</stringValue>
  </field>
  <field>
    <fieldType>1</fieldType>
    <fieldTypeKey>asset_state</fieldTypeKey>
    <stringValue>IDLE</stringValue>
  </field>
  <field>
    <fieldType>1</fieldType>
    <fieldTypeKey>code_out</fieldTypeKey>
    <stringValue>00:00:00.00</stringValue>
  </field>
</response>

I'm trying to extract stringValue where fieldTypeKey == "asset_state":

var response:XML = XML(evt.target.data);
trace("1: "+response.namespace()); 
trace("2: "+getQualifiedClassName(response.namespace()));
trace("3: "+String(response.field.(fieldTypeKey == "asset_state").stringValue));
trace("4: "+getQualifiedClassName(String(response.field.(fieldTypeKey == "asset_state").stringValue)));
trace("5: "+response.field.(fieldTypeKey == "asset_state").toXMLString());
var state:String = String(response.field.(fieldTypeKey == "asset_state").stringValue);
trace("6: state: "+state);

Output:

1: 
2: Namespace
3: IDLE
4: String
5: <field xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY">
  <fieldType>1</fieldType>
  <fieldTypeKey>asset_state</fieldTypeKey>
  <stringValue>IDLE</stringValue>
</field>
TypeError: Error #1010: A term is undefined and has no properties.

TypeError occurs when setting state. I have tried many, many variations of trying to get stringValue as a String, and in every case, it traces out fine, but when I try to set the value to a var, I get the TypeError.

I can see that the namespaces are being added to the field node. My question is, how do I deal with there being no default namespace set? I have tried all manner of setting the default namespace to a new Namespace, getting the response namespace and using that, etc.

I know I could probably get around it by stripping the namespaces out, but that's such a hack, I don't want to do that. Any suggestions?