Are unpaid postdoc positions common?

I contacted a group leader, and he strongly liked my CV. He encouraged me to join his group as a postdoc fellow, but he mentioned that the position he may offer is unpaid (so, I must cover my living expenses).

He mentioned that I can publish good papers in his group following my previous works. Then, I can get a good paid job. So, it is worth of accepting an unpaid job.

I'm wondering if it is common for postdoc fellows to accept unpaid positions?