Error during multiloop step

when trying to call a custom process (to loop and create realted data) within Innovation Suite, the following error presents on save:

ERROR (8790)
Unknown system error. JavaBeanExpressionImpl [beanProperties=[JavaBeanIntegerPropertyImpl [id=536870919], JavaBeanIntegerPropertyImpl [id=379]], contextType=PROCESS_CONTEXT]' didn't resolve to a Collection

A multi instance loop defines process activity repetition. It allows execution of an activity for each item in a data array.
Multi instance loop is available with Call Activity, User Task, Sub-Process, and Service Task Activities.