Trying to get a sum using reduce method

I am trying to get the sum of all (a) properties and console.log says NAN

const numbers = [{ a: 1 }, { a: 6 }, { a: 3 }, { d: 4 }, { e: 5 }, { f: 5 }];

const filterNumbers = numbers.reduce((currenttotal, item) => {
  return item.a + currenttotal;
}, 0);
console.log(filterNumbers);

is there something wrong?