White horizontal line on iMac when used as target display?

Hey everyone,

I am working with a iMac set as a target display with a MacBook Pro (Retina, 2012)

A horizontal white line appears on the iMac—but only when in target display mode.

Does not show up when the screen saver comes up.

Does move when changed to different scaled display resolutions.

Does not show up when the iMac is used out of target display mode.