More than 90% of Fortune 500 companies have mentoring programs, according to a study by MentorcliQ. Mentorships can be a good way to keep employees engaged at work and moving forward in their careers.

Unfortunately, all mentors are not created equal. Since mentoring is almost always voluntary, not everyone who signs up is suited for the role, says Ashton Brown, head of programs and community at Hyper, an early-stage accelerator program.

“Just like not everyone who becomes a manager or leader is experienced in or suited to the role,” she says, adding that a mentor who is a good match for one person can be an imperfect or even toxic-feeling match for someone else.