The most likely cause is the optimization is modifying the expression in an effort to simplify it and the resulting expression is invalid. Can you go to the Developer UI Window menu -> Preferences -> Informatica -> Run Configurations -> Data Viewer -> Advanced and set Optimizer Level to None.
Run a Data Preview on the Expression and see if it runs ok.
Also, what version are you using?
Sorry i haven't had a chance to try this. I've been pulled off of this project. Bus as far as the version, i am using 10.1.1 HF1 Build 55