TypeError: expect(…).to.be is not a function

I am configuring my Karma amd mocha framework with grunt in my project. When I am running karma start I am getting below-mentioned error.

I am getting this error in my console while running command : Karma start

TypeError: expect(...).to.be is not a function

My Karma.confjs

// Karma configuration
// Generated on Fri Nov 27 2015 11:48:47 GMT+0530 (India Standard Time)

module.exports = function(config) {
  config.set({

    // base path that will be used to resolve all patterns (eg. files, exclude)
    basePath: '',


    // frameworks to use
    // available frameworks: https://npmjs.org/browse/keyword/karma-adapter
    frameworks: ['mocha', 'chai'],


    // list of files / patterns to load in the browser
    files: [
      'bower_components/angular/angular.js',
      'bower_components/angular-mocks/angular-mocks.js',
      'app/*.js',
      // 'test/specs/*.js',
      'test/specs/array.js',
      // 'test/specs/myCtlr-spec.js',
      //'test/*.js'
    ],


    // list of files to exclude
    exclude: [
    ],


    // preprocess matching files before serving them to the browser
    // available preprocessors: https://npmjs.org/browse/keyword/karma-preprocessor
    preprocessors: {
    },


    // test results reporter to use
    // possible values: 'dots', 'progress'
    // available reporters: https://npmjs.org/browse/keyword/karma-reporter
    reporters: ['progress','coverage'],

    preprocessors: {
        'src/app/**/*.js': ['coverage']
    },
    coverageReporter: {
        type: 'lcov',
        dir: 'coverage/'
    },

    // web server port
    port: 9876,


    // enable / disable colors in the output (reporters and logs)
    colors: true,


    // level of logging
    // possible values: config.LOG_DISABLE || config.LOG_ERROR || config.LOG_WARN || config.LOG_INFO || config.LOG_DEBUG
    logLevel: config.LOG_INFO,


    // enable / disable watching file and executing tests whenever any file changes
    autoWatch: true,


    // start these browsers
    // available browser launchers: https://npmjs.org/browse/keyword/karma-launcher
    // browsers: ['PhantomJS', 'Chrome'],
    browsers: ['Chrome'],


    // Continuous Integration mode
    // if true, Karma captures browsers, runs the tests and exits
    singleRun: false,

    // Concurrency level
    // how many browser should be started simultanous
    concurrency: Infinity
  })
}

My array.js test

// var expect = require('chai').expect;

describe("Mocha: The 'toBe' matcher compares with ===", function() {
  it("and has a positive case", function() {
    expect(true).to.be(true);
  });

  it("and can have a negative case", function() {
    expect(false).not.to.be(true);
  });
});

Please suggest what I am missing.