I have long suspected this to be true. That's my argument against mandatory voting, because if people can choose not to vote, they can express they don't know and hopefully leave the result on people who feel they do know.

Mandatory voting can include "I don't know" (and, in fact, it naturally falls out in a simple way from any use of preference ballots with a tallying system that accounts for ties and treats all unranked candidates as tied for the position immediately below all explicitly-ranked candidates.)

EDIT: Further, such a system would naturally handle the more common case where people do have known preferences, but they aren't accurately represented either as a forced preference ranking of all candidates (as with preference ballots that reject ties), or a unique most-preferred status for one candidate (as is the case with the "bullet" ballots typically used in FPTP elections.)