Automath breaks background shorthand

[dfltr]

Input:
background: #fff url('some/image.png') no-repeat 50% 30px / 90px 120px;

Output:
background: #fff url('some/image.png') no-repeat 50% .33333333px 120px;

Expected result:
Forward slash in background shorthand should not be treated as division operator, or should be escapable.

For now I'm just avoiding the background-position / background-size shorthand, but this issue does break the CSS spec.