Warning: Failed prop type: c: prop type `size` is invalid;

When using Checkmark as:
```<Checkmark size="xxLarge" />```
or 
```<Checkmark size={96} />```

I get the following error:
```
Warning: Failed prop type: c: prop type `size` is invalid; it must be a function, usually from the `prop-types` package, but received `number`.
```