What Unsupported Operation do you mean?

My application is mysteriously raising the following exception:

[RPC Fault faultString="java.lang.UnsupportedOperationException" faultCode="null" faultDetail="null"]
at mx.data::ConcreteDataService/http://www.adobe.com/2006/flex/mx/internal::dispatchFaultEvent()
at mx.data::CommitResponder/::sendCommitEvent()
at mx.data::CommitResponder/::sendEvents()
at mx.data::CommitResponder/::dispatchFaultEvents()
at mx.data::CommitResponder/result()
at mx.rpc::AsyncRequest/acknowledge()
at ::NetConnectionMessageResponder/NetConnectionChannel.as$37:NetConnect ionMessageResponder::resultHandler()
at mx.messaging::MessageResponder/result()

The exception is being raised by the ActionScript call:
dsItems.fill(oItems, tsgUser.id, tsgSheet.beginDate);
which is the one and only call to this data service in the application.

The data service is instantiated as:
dsItems = new DataService("tsg_itemsvc");

which is configured as:

<destination id="tsg_itemsvc">

<adapter ref="java-dao" />

<properties>
<source>com.trl.flexds.assemblers.TimeItemAssembler</source>
<scope>application</scope>

<metadata>
<identity property="itemId" type="java.lang.Long"/>
</metadata>

<network>
<session-timeout>20</session-timeout>
<paging enabled="false" pageSize="10" />
<throttle-inbound policy="ERROR" max-frequency="500"/>
<throttle-outbound policy="REPLACE" max-frequency="500"/>
</network>

<server>
<fill-method>
<name>fill</name>
<params>java.lang.Integer</params>
</fill-method>
<fill-method>
<name>fill</name>
<params>java.lang.Integer,java.util.Date</params>
</fill-method>
<fill-method>
<name>fill</name>
<params>java.lang.Integer,java.util.Date,java.util.Date</params>
</fill-method>
<fill-method>
<name>fill</name>
<params>java.lang.Integer,java.util.Date,java.util.Date,java.lang.Boo lean</params>
</fill-method>
<sync-method>
<name>syncItem</name>
</sync-method>
</server>

</properties>
</destination>

The assembler class is declared as:
public class TimeItemAssembler extends AbstractAssembler

and implements the following methods:
public Collection fill(Integer iEmpId)
public Collection fill(Integer iEmpId, Date dBegin)
public Collection fill(Integer iEmpId, Date dBegin, Date dEnd)
public Collection fill(Integer iEmpId, Date dBegin, Date dEnd, Boolean lTotal)
public Object getItem(Map oItem)
public void createItem(Object oTarg)
public List syncItem(List oChanges)

The requested fill ultimately executes properly - the items are returned. So what unsupported operation is being attempted? Any help would be appreciated.

Mark