The manuscripts have ἀπιστίη in the nominative (instead of ἀπιστίῃ).  Kahn keeps the manuscript reading, making 'lack of confidence' the subject of the verb.  Diels and others emend it to ἀπιστίῃ in the dative, believing that Plutarch supplies a grammatical subject (that may or may not be different than the one Heraclitus used).