Prefix mapping table must not be empty

Hello,

When I execute my very sample graph using a MultiLevelReader and a XMLWriter with :
<Mapping element="enr" inPort="0" key="Field1">
</Mapping>

(Is it suffisant ?)

in the field "Mapping of port to Xml Structure" of the "Edit Component of XMLWriter", I have this error :
____________________________________________________________________
ERROR [WatchDog] - Graph execution finished with error
ERROR [WatchDog] - Node MULTI_LEVEL_READER0 finished with status: ERROR caused by: Component pre-execute initialization failed. Prefix mapping table must not be empty
ERROR [WatchDog] - Node MULTI_LEVEL_READER0 error details:
Element [MULTI_LEVEL_READER0:MultiLevelReader]-Component pre-execute initialization failed. Prefix mapping table must not be empty
at org.jetel.graph.Node.run(Node.java:418)
at java.lang.Thread.run(Thread.java:619)
Caused by: Prefix mapping table must not be empty
at com.opensys.cloveretl.data.parser.PrefixMultiLevelSelector.a(Unknown Source)
at com.opensys.cloveretl.data.parser.PrefixMultiLevelSelector.init(Unknown Source)
at com.opensys.cloveretl.data.parser.b.init(Unknown Source)
at org.jetel.util.MultiFileReader.init(MultiFileReader.java:108)
at com.opensys.cloveretl.component.MultiLevelReader.preExecute(Unknown Source)
at org.jetel.graph.Node.run(Node.java:416)
... 1 more
INFO [WatchDog] - [Clover] Post-execute phase finalization: 0
____________________________________________________________________

I Have only one field in my input datafile containing one word (the most sample datafile).

Someone can help me ?

Thanks by advance.