why can't i call my invoke function using infix no...

# getting-started

[holgerbrandl]

why can't i call my invoke function using infix notation?

class Dummy <T> (val data: Collection<T>){

  infix operator fun invoke(el:T) : Boolean{
    return data.contains(el)
  }
}

fun main(args: Array<String>) {
  val dummy = Dummy(listOf("a", "b", "c"))

  dummy("a") // works
  dummy "a"   //  does not work
}

From what I understand all 3 conditions for infixing should be met (see https://kotlinlang.org/docs/reference/functions.html#infix-notation)