Method Not Found -- LayerMask.NameToIndex

The documentation tells me I can call LayerMask.NameToIndex('my layer name') to get back an integer (or really LayerMask, I guess). However, when I try to do this, I get the following error:

 MissingMethodException: Method not found: 'UnityEngine.LayerMask.NameToIndex'.

What could be the problem? I'm running version 3.5.6f4 but even the local docs say I should be able to do this.