Why is typeof(readln) -> void?

[blizzard via Digitalmars-d-learn]

```
import std.stdio;
void main()
{
    writeln(typeof(readln()).stringof); // string
    writeln(typeof(readln).stringof); // void
}
```
Can anybody explain the difference between readln() and readln? I 
read somewhere that the () were optional, so why this difference?