grunt-contrig-imagemin Warning: Cannot call method 'replace' of undefined

I'm getting a weird warning when trying to use imagemin and nothing happens. ``` grunt imagemin:primary Warning: Cannot call method 'replace' of undefined Used --force, continuing. Done, but with warnings. ``` My task that gives me the error is as follows ``` grunt.config 'imagemin' primary: options: optimizationLevel: 7 expand: true cwd: "assets" src: "{img,img_two}/**/*.{jpg,jpeg,gif,png}" dest: "public/compiled/img" grunt.loadNpmTasks 'grunt-contrib-imagemin' ``` These don't work `src: "img/**/*.{jpg,jpeg,gif,png}"` `src: "{img,img_two}/**/*.{jpg,jpeg,gif,png}"` These work `src: "img/**/*.jpg"` `src: "{img,img_two}/**/*.jpg"` So it seems it doesn't like multiple extensions syntax but why, it's even how it's done in the example? UPDATE: it seems it's just PNG that doesn't work whether I do the extension on it's own or within a group matching. I'm on Windows 8, NPM 1.4.3 https://github.com/gruntjs/grunt-contrib-imagemin/issues/219