[flexcoders] error in HTTPService

[weezee49]

I have the following HTTPService:

<mx:HTTPService id="svcSimulationUpdate" 
    contentType="application/xml" 
    url="http://localhost:3000/simulations/
{userModelsCB.selectedItem.id}?_method=put"
    resultFormat="e4x" 
    method="POST" 
    result="handleSimUpdateResult(event)">
    <mx:request xmlns="">
    <data>
        <simulation>
            <name>{runControlBox.simNameTI.text}</name>
            <desc>{runControlBox.descTI.text}</desc>
            <run_length>{runControlBox.runLengthTI.text}</run_length>
            <time_units>{getTimeUnits
(runControlBox.timeUnitsCB.selectedIndex)}</time_units>
        </simulation>
        <load>
            <simulation_id>{_selectedSim.id}</simulation_id>
            <name>Arrival1</name>
            <dist_type>{getDistName
(loadBox.loadDistCB.selectedIndex)}</dist_type>
            <param1>{loadBox.loadParam1TI.text}</param1>
            <param2>{loadBox.loadParam2TI.text}</param2>
            <param3>{loadBox.loadParam3TI.text}</param3>
            <time_units>{getTimeUnits
(loadBox.timeUnitsCB.selectedIndex)}</time_units>
        </load>
        <resource>
            <name>Server1</name>
            <simulation_id>{_selectedSim.id}</simulation_id>
            <capacity>{rscBox.capacityTI.text}</capacity>
            <dist_type>{getDistName
(rscBox.distributionCB.selectedIndex)}</dist_type>
            <param1>{rscBox.distParam1TI.text}</param1>
            <param2>{rscBox.distParam2TI.text}</param2>
            <param3>{rscBox.distParam3TI.text}</param3>
            <time_units>{getTimeUnits
(rscBox.timeUnitsCB.selectedIndex)}</time_units>
        </resource>
        <buffer>
            <name>Queue1</name>
            <simulation_id>{_selectedSim.id}</simulation_id>
            <capacity>{queueBox.capacityTI.text}</capacity>
        </buffer>
    </data>
    </mx:request>
</mx:HTTPService>

This service works perfectly the first time - my data base is 
updated correctly and everything is fine.  However, if I try to run 
the service again, I get a "Thread suspended" with an error that 
starts out like this:

undefined
        at flash.xml::XMLNode()
        at flash.xml::XMLDocument/createElement()
        at mx.rpc.xml::SimpleXMLEncoder/encodeValue()
[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\xml\SimpleXMLEncoder
.as:133]
        at mx.rpc.xml::SimpleXMLEncoder/encodeValue()
[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\xml\SimpleXMLEncoder
.as:146]
        at mx.rpc.xml::SimpleXMLEncoder/encodeValue()
[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\xml\SimpleXMLEncoder
.as:146]
        at mx.rpc.xml::SimpleXMLEncoder/encodeValue()
[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\xml\SimpleXMLEncoder
.as:146]
...     

I have verified that the fields used in the <mx:request> have the 
correct data in them just before the send() call.

Why does the service work okay the first time but then die the 2nd 
time?

Thanks,
LG Rains