Getting wrong value in variable after execution of IIFE (with a function parameter) [duplicate]

In this example, I am passing foo.bar to an IIFE, which returns executing 1st-argument. I am passing a function which gets executed, returning this (pointing to window).

So, the return is stored in a which should be window. Then, why it is returning me the argument itself?

var foo = {
  bar: function() { 
    console.log("***", this);
    return this; 
  },
  baz: 1
};

var a = (function(){
  console.log("window?", this===window);
  return arguments[0](); 
})(foo.bar);

console.log("a", a);