Underscore in List.filter

Why this is not working:

List(true,false).filter(_).size

The error says:

<console>:8: error: missing parameter type for expanded function 
((x$1) => List(true, false).filter(x$1).size)
    List(true,false).filter(_).size
                            ^

but the following works (looks just the same for me):

List(true,false).filter(a => a).size

I'm using Scala 2.9.0.1.