Nested calls doesn't return the value

When I use one function inside another the font size doesn't return properly:
```
font-size: ${size(prop('fontSize'))};
```

But when I use only one of the functions the return is correct:

```
font-size: ${({ fontSize }) => key(['sizes', fontSize])};
```

This is the intended way to use the lib or its some kind of bug? I analyzed the return from prop function and see that is a function, can this be the problem that can't be used in nested calls?