alla breve for time 2/1

[Kris Van Bruwaene]

I want to use an alla breve or barred C for time 2/1. I found this 
snippet on a previous post:
\set Staff.timeSignatureFraction = #'(2. 2)
but when I try it it gives an error in my Lily version (2.16.1):
waarschuwing: type controle gefaald voor `timeSignatureFraction'; waarde 
`(2.0 2)' moet type hebben: `fraction, as pair'
??