Having started to look at optimization of queries, I believe this query can be reduced to "true" avoiding the expected error.

Since (/) introduces a "treat as document", the static type of the argument to fn:root can be deduced as empty-sequence, because we know that the parent of a document node is empty-sequence.  fn:root therefore returns empty-sequence, then empty() returns true.

I believe this to be similar to the case pointed out by Mike Kay in bug report 4277 (in which static type analysis means that a cast which would fail need not be evaluated).

(:*******************************************************:)
(: Test: K2-Axes-32                                      :)
(: Written by: Frans Englich                             :)
(: Date: 2007-01-22T21:16:10+01:00                       :)
(: Purpose: Apply axis '..' to the return value of fn:root(). :)
(:*******************************************************:)
declare variable $myVar := <e/>;
            empty(fn:root($myVar/(/)/..))