'CSS variables breaking ASP.Net minification

We use the APS.Net bundling to minify our css files. We also include an external .min file in the bundle. We cannot exclude this file. We need to be include them as we have the our overrides for some of the css in the external file that we put in our file. However the latest version of this external file has introduced CSS variables which is breaking the minifications. It seems like the Does anybody know how to deal with this? We use Microsoft.AspNet.Web.Optimization.1.1.3



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