Converting negative z-index to positive on minification

[binho500]

When minifying .CSS the engine converts negative z-Index to positive.
Example:
Original CSS:

.backLogo{
    position:absolute;
    bottom: 10px;
    right:15px;
    opacity: 0.4;
    z-index: -1;
}

Minified:
.backLogo{position:absolute;bottom:10px;right:15px;opacity:.4;z-index:1}

According to W3C, negative z-Index are allowed:

number Sets the stack order of the element. Negative numbers are allowed

http://www.w3schools.com/cssref/pr_pos_z-index.asp

[XhmikosR]

Such issues should be reported upstream.