p:viewport-source - err:XD0006 or err:XD0003?

Section 5.3 (p:viewport-source) says: "It is a dynamic error
(err:XD0006) unless exactly one document appears on the
p:viewport-source. No select expression is allowed."

Shouldn't the error be err:XD0003 instead of err:XD0006?

Regards,
Vojtech