TypeError: Unable to get property 'setPromocodeDetails' of undefined or null reference

I have defined my service as

angular.module('myservice',[]).service('dataService', function () {
  var promocodeDetails = {
    "havePromocode": false,
    "promocode": "",
    "OfferHeading": "",
    "OfferDescription": "",
    "OfferDisclaimer": ""
  };
  return {
      setPromocodeDetails: function (promocode) {
      promocodeDetails = promocode;
    },
    getPromocodeDetails: function () {
        return promocodeDetails;
    }
  };
});

I have written jasmine test for it

describe('Testing dataservice - a service', function(){
beforeEach(function(){
        module('myservice');
});
var dataService;
beforeEach(inject(function(_dataService_){
    dataService = _dataService_;


}));
it('should set the promocodedetail', function(){
    var promocode={
        'promocode':'12345'
    }
    dataService.setPromocodeDetails(promocode);
    expect(dataService.getPromocodeDetails().promocode).toEqual('12345');
  });
});

When I try to run this test, I get the following error

TypeError: Unable to get property 'setPromocodeDetails' of undefined or null    reference

Please suggest as I am stuck with this simple test...