incf in a let form
In common lisp, we are allowed to incf in a let form as per these examples (link: https://jtra.cz/stuff/lisp/sclr/incf.html):

```
(let ((a 10)) (incf a) a) => 11
(let ((a 10)) (incf a 2.3) a) => 12.3
(let ((a 10)) (incf a -2.3) a) => 7.7
(let ((a (list 10 11 12 13))) (incf (elt a 2) 2.3) a) => (10 11 14.3 13)
```

However, when I have a let form in Sketch and I incf a lexically scoped variable, I get the following error:

```
Error - The value NIL is not of type (UNSIGNED-BYTE 32) when binding SB-ALIEN::VALUE
```
The code:
```
(dotimes (x 10)
  (dotimes (y 10)
    (with-pen (make-pen :fill (let ((x 0.01))
                                (rgb x x x) 
                                (incf x 0.01))
                        :stroke (gray 0.2)
                        :weight 11)
      (rect (* x 80) (* y 80) 80 80))))
```

Does anyone know why the (incf) in the let triggers the unsigned-byte 32 error?
