Source map not being created

[sethreidnz]

Hi,

I am using concat with usemin and this is my configuration:

concat: {
       options : {
        sourceMap :true
      }
    },

    uglify: {
      options: {
        sourceMap: true,
        sourceMapIn: function(uglifySource) {
          return uglifySource + '.map';
        },
        sourceMapIncludeSources: true
      }
    },

However concat fails to actually create the map file so uglify cannot find it. Any ideas why setting sourceMap to true doesn't work?