Use the following rules and guidelines for expression masking:
You cannot use the output from an expression as input to another expression. If you manually add the output port name to the expression, you might get unexpected results.
Use the point-and-click method to build expressions. Select functions, ports, variables, and operators from the point-and-click interface to minimize errors when you build expressions.
If the Data Masking transformation is configured for repeatable masking, and the storage table does not exist, the Integration Service substitutes the source data with default values.