"Process failed: Exception: java.lang.NullPointerException in Aggregate"

Hi everyone! I'm new to RapidMiner (and data mining) so I hope you could help me.

I'm trying to get "Aggregate" to work but I get an error whenever I try to group by attributes. I get this error:


[tt]Exception: java.lang.NullPointerException
Message: null
Stack trace:

 com.rapidminer.operator.preprocessing.transformation.aggregation.AggregationOperator$AggregationTreeNode.getChilds(AggregationOperator.java:108)
 com.rapidminer.operator.preprocessing.transformation.aggregation.AggregationOperator.parseTree(AggregationOperator.java:394)
 com.rapidminer.operator.preprocessing.transformation.aggregation.AggregationOperator.apply(AggregationOperator.java:324)
 com.rapidminer.operator.AbstractExampleSetProcessing.doWork(AbstractExampleSetProcessing.java:116)
 com.rapidminer.operator.Operator.execute(Operator.java:831)
 com.rapidminer.operator.execution.SimpleUnitExecutor.execute(SimpleUnitExecutor.java:51)
 com.rapidminer.operator.ExecutionUnit.execute(ExecutionUnit.java:709)
 com.rapidminer.operator.OperatorChain.doWork(OperatorChain.java:369)
 com.rapidminer.operator.Operator.execute(Operator.java:831)
 com.rapidminer.Process.run(Process.java:902)
 com.rapidminer.Process.run(Process.java:798)
 com.rapidminer.Process.run(Process.java:793)
 com.rapidminer.Process.run(Process.java:783)
 com.rapidminer.gui.ProcessThread.run(ProcessThread.java:63)[/tt]

But when I remove all entries in "group by attributes" it works fine. Anyone know what I'm doing wrong?   Thanks!