How can I send a message from one controller to another in AngularJS?

I have the following set up:

stApp.controller('AdminTableController', ['$rootScope', '$scope', 'gridService', 
            function ($rootScope, $scope, gridService) {
    $scope.$watch('tableData.$pristine', function (newValue) {
        $rootScope.broadcast("tableDataUpdated", {
            state: page.$pristine
        });
    });
}])

stApp.controller('AdminGridController', ['$rootScope', '$scope', 'gridService',
            function ($rootScope, $scope, gridService) {
    $rootScope.on("tableDataUpdated", function (args) {
        //args.state would have the state.
        alert(args.state);
    });
}])

When I run this code I am getting a message:

Object #<Object> has no method 'on'

Note that I tried this with both $rootScope.on and $scope.on