White Screen of Death when I Optimize CSS

Apologies if this is in the wrong place, I'm kinda new to this.

I'm using a custom Zen theme I built using Zenophile. If I optimize the CSS files and Save configuration, I'm greeted with the WSOD.

I got around this by renaming my custom theme's main folder so that the CSS can't be found. I can then navigate back to the Performance page and choose not to optimize CSS, then rename the theme folder name correctly.

Any ideas?