                 ).equal right[resultPath].content
             )
             promises.push q.nfcall(
-              fs.readFile,
+              fs.readFile.bind fs.readFile,
               resultPath + ".map"
             ).then(
               (map) ->

    before (done) ->
      targetPromises = [
        q.nfcall(
          fs.readFile,
          "./tests/data/single/source.css"
        ).then(
          (css) -> right.file = removeMapFile(css.toString "utf-8").trim()
        )
        q.nfcall(
          fs.readFile,
          "./tests/data/single/source.css.map"
        ).then(
          (map) ->
            right.sourcemap = JSON.parse(map.toString "utf-8")
            right.sourcemap.sources = right.sourcemap.sources.map(
              (file) -> path.relative(
                process.cwd(), path.resolve "./tests/data/single", file

            ).trim()
        )
        promises.push q.nfcall(
          fs.readFile.bind fs.readFile,
          path.join("./tests/data/multiple", file + ".map")
        ).then(
          (data) ->
