Is the federal habeas custody requirement met where a petitioner makes a constitutional challenge to a previously unchallenged conviction that was used to enhance the sentence the petitioner is currently serving if the failure to challenge the earlier conviction was through no fault of the petitioner?

QUESTION PRESENTED Is the federal habeas custody requirement met where a petitioner makes a constitutional challenge to a previously unchallenged conviction that was used to enhance the sentence the petitioner is currently serving if the failure to challenge the earlier conviction was through no fault of the petitioner?