Modifying Custom Fields using PSI

I am trying to update Custom Fields using PSI, I am ending up in exception... 

System.InvalidOperationException: Collection was modified; enumeration operation might not execute.
   at System.Data.RBTree`1.RBTreeEnumerator.MoveNext()
   at System.Linq.Enumerable.<CastIterator>d__aa`1.MoveNext()

Any ideas ???