Keyboard needs to be plugged in

[jaro3441]

El Capitän on iMac and Macbook Pro

iMac, 27-inch, Mid 2011
3,4 GHz Intel Core i7
16 GB 1333 MHz DDR3

MacBook Pro Retina, 15 Inch, mid 2015
2,8 GHz Intel Core i7
16 GB 1600 MHz DDR3

We need the keyboard plugged in to get this going. Is this a known issue?
What to do?

Good fix, but annoying with keyboard requirement.

[rinopo]

Some people are talking about the need of a keyboard in the comments to the original blog post.
https://bogner.sh/2013/07/os-x-how-to-use-an-imac-as-monitor/#comment-54248
https://bogner.sh/2013/07/os-x-how-to-use-an-imac-as-monitor/#comment-63537