Sprites with individual spacing renders white background

[cscheng]

When I create a sprite that has spacing, it renders the entire sprite with a white background.

For example:

$icon: sprite-map('icon/*.png', $position: 0%, $spacing: 100px, $repeat: no-repeat);
@import 'icon/*.png';

However, the transparency of the individual images is maintained. Only the background of the spacing is white. It occurs with 8-bit PNGs, 24-bit PNGs, and I have also tried it with mixed bit depths.

Why does this happen? As a workaround I'm using the default sprite engine, which renders the spacing correctly (transparent). But it's really slow...