Unexpected error when using appendChild() in SDK 3.4 and above

Question

I'm using an API class to build XML objects within AS3 and passing them as a String (of well-formed XML) to a Web service. This class has worked just fine in Flex 3.0:
public class MyXMLClass
private var message:XML = <root></root>;
public function MyXMLClass()
message.appendChild(<arguments></arguments>);
public function setArgument(name:String):void
message.arguments[0].appendChild("<argument name=\"" + name +"\"></argument>");
public function setArgumentValue(name:String,value:String):void
message.arguments[0].argument.(@name == name).appendChild("<value>" + value + "</value>");
public function getMessage():XML
return message;
//Take the result of getMessage().toXMLString() and pass that into the Web service as a String.
But now that I am running Flash Builder whether I compile with 3.4 or 4.0 SDK I get:
Error #1086: The appendChild method only works on lists containing one item.
The line of code it chokes on is the setArgumentValue function.    

Answer 1

Yeah that was it Alex. The following code change worked:
public function setArgument(name:String):void
message.arguments[0].appendChild(<argument name={ name }></argument>);
public function setArgumentValue(name:String,value:String):void
message.arguments[0].argument.(@name == name).appendChild(<value>{ value }</value>);
This now works in SDK 3.4. Unfortunately SDK 4.0 (Adobe Forums: Discrepancy in XML between Flex 3.4 and...) breaks all my XML but at least I can cross this one off. Thanks.
11/15/09: Glad to report having found a workaround for the other issue mentioned above.