Array concatenating adds undefined

[makmonty]

When I merge two objects with an array attribute, the resulting object contains that array merged but with an extra undefined element:

var extend = require('extendify')({arrays: 'concat'});
var obj = extend({}, {a: [1,2]}, {a:[3,4]});
console.log(obj);
-> { a: [ undefined, 1, 2, 3, 4 ] }