This is most likely a minifier bug - assuming you are using the default (AjaxMin) you might want to try using YUI?
In global.asax.cs (or startup if using OWIN) you can execute the following to change the default minifier:
Bundle.ConfigureDefaults().UseYuiForCssMinification();
This will be changing in a future release but should work for now. There are also .WithMinifier methods you can use to change at the individual bundle level.
Hope this helps
Alex