TypeError: string is not a function. In unit test

I am new javascript. After I run my unit test. I got an error.

Test with static input
1) should return some1

0 passing (6ms)
1 failing

1) Test with static input should return some1:
 TypeError: string is not a function
 at Object.mock.fs.readFile (test/m1.js:9:14)
 at Object.exports.readFile (arn.js:10:19)
 at Context. (test/m1.js:24:5)

---

Here is my code .test/m1.js

var mock = require('mock')
var realB = require("../arn.js")
var test = require('unit.js');

var b = mock("../arn.js", {
    fs: {
        readFile: function (entry, callback) {
            if (entry === "clowns.txt") {
                callback(null,'some1');
            }
        }
    }
}, require);

describe('Test with static input', function(){
    it('should return some1', function(done){
        b.readFile('clowns.txt', function(a){
            test.value(a).match('some1');
            done();
        });
    });
});

---

./arn.js

var fs = require('fs');

 exports.readFile = function(entry,callback){

     var cntent = fs.readFile(entry, "utf8", function(err, data) {
              if (err) throw err;
              //console.log(entry + " : " +  data);
              callback(data.substring(0,5));
     });
 }

I am not sure where I miss. Any help would be appreciated.
Sarit