Saxon

Indeed, the error is that the function supplied as the second argument of the call to filter must not return an empty sequence.

A map of course is a function, and when it's supplied to fn:filter, function coercion happens, so the call on the map function gets wrapped in an anonymous function that checks the result. On this occasion the check fails, and the challenge is how to interpret this in terms that the user might understand.

The relevant code is at CoercedFunction#194

Supplier<RoleDiagnostic> role =
                    () -> new RoleDiagnostic(RoleDiagnostic.FUNCTION_RESULT, targetFunction.getDescription(), 0);

Instead of using targetFunction.getDescription() we should construct a more detailed description.

A bit of tweaking gives the message

XPTY0004  An empty sequence is not allowed as the result of a call to map{} (used where
  the required type is (function(item()) as xs:boolean))

The changes made are:

(1) CoercedFunction.getDescription() changed to return

return targetFunction.getDescription() + " (used where the required type is " + requiredType + ")";
           

(2) the code shown above changed to return getDescription() rather than targetFunction.getDescription()

(3) MapItem.getDescription chaged to return toShortString() rather than "map".