During creating and testing your jPDL processes it will be great feature having ability to tell jBPM:
"When process evaluating EL and accessing variable does not exists RAISE EXCEPTION"

Advantages:

• Your process fail in place where is problem (e.g. you misspell variable name)
• Faster error finding, because of break early

I propose to make configuration variable for this option, because of back compatibility, but in future it should be default.

Now jBPM returns null if we evaluate non existsing variable, which is error prone.