Motion with dynamic properties

Hi, great lib, I have been using this for a while. But now I'm facing a issue, I may miss something :)

I have to interpolate 2 properties, for example:

````
const styles = {
x: spring(x),
y: spring(x),
};

{interpolatedStyles => (

)}

````

If I **remove y** and **add z**
````
const styles = {
x: spring(x),
z: spring(z),
};
````

then when I try to get **interpolatedStyles.z** I got **NaN** :(
I noticed that on the first call of the interpolation callback rendering I still find **y** property too
I tried to give a defaultStyle that also have the updated property but I get the same result with z = NaN

Thank you for your help :)