Saxon

If a filter expression after optimization reduces to something like (1 to 20)[89], where both operands are literals and the index is out of range for the length of the sequence, a NullPointerException occurs rather than returning the empty sequence.

Problem code is at line 1121 in FilterExpression.iterate()

return ((Literal)base).getValue().itemAt(pos - 1).iterate();

where itemAt() can return null.

There is also another instance a few lines earlier where the base expression is a variable reference.