Running su with -c gives unrecognized command error

I am trying to run a command as another user in Linux. Here is what I run:

su user1 -c '/bin/mkdir /tmp/zz'

However, this gives me this error:

fatal: unrecognized command '/bin/mkdir /tmp/zz'

You need to swap the single quotes for double quotes. Not that these are only required with commands that take parameters, or if you chain several commands together.

su user1 -c "/bin/mkdir /tmp/zz"