can I not project into the group tuple from FILTER?

[Daniel Eklund]

If I can access the implicit 'group' column from within FOREACH like this:

GROUPED = GROUP InputRelVar by (firstDim,secondDim);
B = FOREACH GROUPED  GENERATE   group.firstDim;

... then should I not be able to do something like this?

B1 = FILTER GROUPED by group.firstDim == 'something';

I get messages like this:
java.lang.ClassCastException: java.lang.String cannot be cast to
org.apache.pig.data.Tuple
    at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POProject.getNext(POProject.java:392)
    at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.PhysicalOperator.processInput(PhysicalOperator.java:276)
    at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POProject.getNext(POProject.java:138)
    at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POProject.getNext(POProject.java:276)
    at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POIsNull.getNext(POIsNull.java:72)
    at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.PONot.getNext(PONot.java:71)
    at
org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POFilter.getNext(POFilter.java:148)

Interestingly I can use the 'group' alias overall like
B2 = FILTER GROUPED by group is not null;


Any explanations of what I am doing incorrect here?

thanks,
daniel