Concatenate Strings one or more of which may be null

Hi,

I have strings A, B, C, D, E, F, G H
from Table Input where there may be one or more null, I want to output one string A&B&C&...
I am trying to concatenate strings in Formula step using "&" as an operator


I get this error when one of the strings is null
2014/02/25 15:53:25 - Formula.0 - ERROR (version 4.1.0-stable, build 14410 from 2010-11-16 16.43.28 by buildguy) : Unexpected error :
2014/02/25 15:53:25 - Formula.0 - ERROR (version 4.1.0-stable, build 14410 from 2010-11-16 16.43.28 by buildguy) : org.pentaho.di.core.exception.KettleValueException:
2014/02/25 15:53:25 - Formula.0 - ERROR (version 4.1.0-stable, build 14410 from 2010-11-16 16.43.28 by buildguy) : org.pentaho.reporting.libraries.formula.EvaluationException: LibFormulaErrorValue{errorCode=522, errorMessage=NA}
2014/02/25 15:53:25 - Formula.0 - ERROR (version 4.1.0-stable, build 14410 from 2010-11-16 16.43.28 by buildguy) : LibFormulaErrorValue{errorCode=522, errorMessage=NA}
2014/02/25 15:53:25 - Formula.0 - ERROR (version 4.1.0-stable, build 14410 from 2010-11-16 16.43.28 by buildguy) : org.pentaho.di.trans.steps.formula.Formula.calcFields(Formula.java:230)
2014/02/25 15:53:25 - Formula.0 - ERROR (version 4.1.0-stable, build 14410 from 2010-11-16 16.43.28 by buildguy) : org.pentaho.di.trans.steps.formula.Formula.processRow(Formula.java:97)
2014/02/25 15:53:25 - Formula.0 - ERROR (version 4.1.0-stable, build 14410 from 2010-11-16 16.43.28 by buildguy) : org.pentaho.di.trans.step.RunThread.run(RunThread.java:40)
2014/02/25 15:53:25 - Formula.0 - ERROR (version 4.1.0-stable, build 14410 from 2010-11-16 16.43.28 by buildguy) : java.lang.Thread.run(Thread.java:662)

how can I get around this issue?
Thanks,
Rod