fs.exists not accepting variables

I'm using post requests to search for binaries but can't get the following code to work correctly. fs.exists() won't accept the variable b_path but will work correctly if given a hard coded string. b_path prints to the console as expected, correctly building the path to the binary.

app.post('*', function(req, res) {

  // generate the name of the binary
  var request = require('url').parse(req.url, true);
  var len = request.pathname.toString().length;

  var binary = request.pathname.slice(1,len);
  binary = binary.concat(' ');

  var b_path = app.get('binaries_path')+binary;

  fs.exists(b_path, function (exists) {
    if(exists) {
      console.log('exists');
    }
  }
}

Why does this occur?