Error message btree: BTP_CHAIN

Can someone explain this error I get in my postgres.log while trying to run
a simple query (select * from table) on a table that has this index:

FATAL 1:  btree: BTP_CHAIN flag was expected in site_login_user (access =
bt_read)

What is the BTP_CHAIN and how did it get set?  I'm trying to figure out
what the user could have done to cause this (and how I can fix it.)

Thank you,
Matthew Hagerty