What type is the function input to String.withCString()?

I'm using the function String.withCString() as follows:

let s = "Hey!"let c = s.withCString {    strlen($0)}println(c)

However, if I add a second line to the withCString closure, I get an error

let s = "Hey!"let c = s.withCString {    println("test")    strlen($0)}println(c)

The error is: "Cannot convert the expression's type '(($T4) -> $T3) -> (($T4) -> $T3) -> $T3' to type 'Result'.

Looking at the declaration for withCString it says:

func withCString<Result>(f: (UnsafePointer<Int8>) -> Result) -> Result

Which I have been interpreting as saying that the f parameter will be a function that takes an UnsafePointer<Int8> as input and returns some type as a result. This is what my second example is doing. What's wrong with the second example block?