A brief post to capture the fact that you may get ambigous exception using the update multiple item activity.
Setting fields to current list item fields which have no value. Or setting no value on the field will effectively throw a null reference exception. I wanted to create a single activity to set all the fields to the parent item fields but this failed as the step continues to throw null reference exceptions. If the user inputs no value into the field this can also trigger the exception, even if the field is an optional field.
The trick is to only set fields you have recieved data for.