R.length not returning NaN

Hi, 

For below objects, R.length is not returning NaN
```
console.log( R.length({length: ''}));
console.log( R.length({length: '1.23'}));
console.log( R.length({length: null}));
```
output:
```

1.23
null
```
expected:
```
NaN
NaN
NaN
```