overloading <( )> error without line number

gistfile1.txt

use v6;

class Signal {
  has @.slots is rw;

  multi method connect(Code $f){
    @.slots.push($f);

    return self;
  }

  multi method connect($obj, $f){
    @.slots.push(sub ($c){$obj.$f(|$c)});

    return self;
  }

  multi method postcircumfix:<( )>(){
    $.slots>>.(); # HERE BE DRAGONS
  }
}

### output:
### ===SORRY!===
### Method 'isa' not found for invocant of class 'Undef'