sourceMap.length undefined

[Deleted user]

I'm getting the following error when using autoprefixer-core:

Error: "name" and "value" are required for setHeader().
    at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:333:11)
    at ServerResponse.res.setHeader (/projects/twd/node_modules/loopback-explorer/node_modules/express/node_modules/connect/lib/patch.js:134:22)
    at Assets.serveAsset (/projects/twd/node_modules/connect-assets/lib/assets.js:94:11)
    at serve (/projects/twd/node_modules/connect-assets/index.js:36:23)
    ...

Is this any issue with mincer and the change from autoprefixer and autoprefixer-core?
Going to /node_modules/connect-assets/lib/assets.js:94:11 and debugging I found that the sourceMap.length property is undefined.

Note: You may see that i'm using loopback framework with connect-assets, if it is a useful information.