Cannot read WI_OBJECT_ID

  Dear Sir,

1. I have a workflow trigger through an event.

on the events, the data binding is

&_EVT_OBJECT& = &OBJ&

2. I create an activity with data binding

&OBJ& = &WI_OBJECT_ID&

In this step I can read all the data in the WI_OBJECT_ID

3. I create another activity with data binding

&OBJ& = &WI_OBJECT_ID&

BUT when I try to read the variable in WI_OBJECT_ID

I got an error

Variables of the work item text cannot be generated

Error in the evaluation of expression '<???>&_WI_OBJECT_ID.EMPLOYEENUMBER&' for item '1'

Why I can't read the object in step 3?

Thanks

Sam