Cannot set property '01' of undefined

I have this code:

var _ = require('underscore');

var myObj = [

    {'name': 'Mike', 'number' : 'b1' , 'level' : 0 },
    {'name': 'Tom', 'number' : 'b2' , 'level' : 0 }
];

var result = _.map(myObj, _.invert);

Now, the result is:

[ { '0': 'level', Mike: 'name', b1: 'number' },
  { '0': 'level', Tom: 'name', b2: 'number' } ]

Ok, until here.

Then, I am adding to the result some parts and trying to fill them:

result = result,
    parts = {'levels': {}, 'names': {}, 'numbers': {}};

result.forEach(function(obj,i){

    Object.keys(obj).forEach(function(k) { 

        parts[k +"s"][k +(i+1)] = obj[k];
    });
});

but I am receiving :

TypeError: Cannot set property '01' of undefined

at the parts[k +"s"][k +(i+1)] = obj[k];

Just note ,that if I don't use the invert , code runs without problems.