Unable to subscribe from a topic.

I have following code in my app.js,

$scope.subscribeTopic = function() {
    $scope.subscription = $wamp.subscribe($scope.topic, function(args, kwargs, details){
        console.log(update);
    });
}

$scope.unsubscribeTopic = function() {
    $scope.subscription.unsubscribe();
}

unsubscribeTopic method is tied to a buttton, but when I click on this button I get following error message,

TypeError: $scope.subscription.unsubscribe is not a function
    at a.$$childScopeClass.$$childScopeClass.$scope.unsubscribeTopic (app.js:51)
    at angular.js:10847
    at f (angular.js:19136)
    at a.$$childScopeClass.$$childScopeClass.$eval (angular.js:12702)
    at a.$$childScopeClass.$$childScopeClass.$apply (angular.js:12800)
    at HTMLButtonElement.<anonymous> (angular.js:19141)
    at HTMLButtonElement.dispatch (jquery-min.js:3)
    at HTMLButtonElement.i (jquery-min.js:3)
(anonymous) @ angular.js:10072
(anonymous) @ angular.js:7364
$apply @ angular.js:12802
(anonymous) @ angular.js:19141
dispatch @ jquery-min.js:3
i @ jquery-min.js:3

am I doing something wrong?

EDIT

I tried with following code as well,

    $scope.unsubscribeTopic = function() {
        $wamp.unsubscribe($scope.subscription);
    }

but then I get following error,

TypeError: a.unsubscribe is not a function
    at angular-wamp.min.js:1
    at angular-wamp.min.js:1
    at F (angular.js:11573)
    at angular.js:11659
    at k.$eval (angular.js:12702)
    at k.$digest (angular.js:12514)
    at a.$$childScopeClass.$$childScopeClass.$apply (angular.js:12806)
    at HTMLButtonElement.<anonymous> (angular.js:19141)
    at HTMLButtonElement.dispatch (jquery-min.js:3)
    at HTMLButtonElement.i (jquery-min.js:3)
(anonymous) @ angular.js:10072
(anonymous) @ angular.js:7364
F @ angular.js:11576
(anonymous) @ angular.js:11659
$eval @ angular.js:12702
$digest @ angular.js:12514
$apply @ angular.js:12806
(anonymous) @ angular.js:19141
dispatch @ jquery-min.js:3
i @ jquery-min.js:3
angular.js:10072 $wamp error Object {type: "unsubscribe", arguments: Arguments[1], error: TypeError: a.unsubscribe is not a function
    at http://localhost:8080/myapp/resources/js/an…}

voryx / angular-wamp

Unable to subscribe from a topic. #59