Problem with a condition calculation

HI All

I'm having another problem with a simple calculation in a pre-condition:

import System
static def GetAttributeValue(Request):
Value = 'False'
for item in Request.Tasks:
  if item.Category.Title == 'Build Task' and item.Status.Title == 'Completed':
   Value = 'True'
return Value

I use this precondition to setup mulitple parallel and seiral tasks.  However, when this pre-condition is used after several add tasks and when the other tasks are completed, then I get an unusal error where the following automatic action add task after the pre-condition suddenly becomes an action instead.

Once I remove the calcultion condition then everything flows ok so I'm assuming it's the calculation that's causing my issues.