usemin fails with Arguments to path.join must be strings

Solution

Wrong :

<!-- build:css -->
 <link href="css/base.css" rel="stylesheet" />
 <link href="css/appname.css" rel="stylesheet" />
 <!-- endbuild -->

Good :

<!-- build:css index.css -->
 <link href="css/base.css" rel="stylesheet" />
 <link href="css/appname.css" rel="stylesheet" />
 <!-- endbuild -->

Error :

path.js:204
 throw new TypeError('Arguments to path.join must be strings');
 ^
 TypeError: Arguments to path.join must be strings
 at f (path.js:204:15)
 at Object.filter (native)
 at Object.exports.join (path.js:209:40)
 at createFile (C:\dev\sandbox.svn\ebay-rock-paper\node_modules\gulp-usemin\index.js:20:25)
 at concat (C:\dev\sandbox.svn\ebay-rock-paper\node_modules\gulp-usemin\index.js:76:12)
 at processTask (C:\dev\sandbox.svn\ebay-rock-paper\node_modules\gulp-usemin\index.js:83:19)
 at processTask (C:\dev\sandbox.svn\ebay-rock-paper\node_modules\gulp-usemin\index.js:100:7)
 at process (C:\dev\sandbox.svn\ebay-rock-paper\node_modules\gulp-usemin\index.js:110:5)
 at processHtml (C:\dev\sandbox.svn\ebay-rock-paper\node_modules\gulp-usemin\index.js:137:13)
 at Transform._transform (C:\dev\sandbox.svn\ebay-rock-paper\node_modules\gulp-usemin\index.js:171:7)