$routeParams gets undefined in angularjs?

I am making a call an http call to a json file. In the first controller I get the categories , and in the second controller I get the category with a certain ID.

The url gets updated with an categoryid but the $routeParams is shown undefined and cannnot access the file.

Here is my code:

$stateProvider.state('categories', {
  url: '/category',
  templateUrl: 'templates/categories.html',
  controller: 'CategoriesCtrl'
});
$stateProvider.state('postC', {
  url: '/category/:category',
  templateUrl: 'templates/postsc.html',
  controller: 'PostCtrl'
});

In the controller I have the following:

angular.module('myapp')

.controller('CategoriesCtrl', ['$scope', '$http',
  function($scope, $http) {
    $http.get("~/wp/v2/terms/category")
      .then(function(res) {
        $scope.categories = res.data;
      })
  }
])

.controller('PostCtrl', ['$scope', '$http', '$routeParams',
  function($scope, $http, $routeParams) {
    $http.get("~/wp/v2/terms/category/" + $routeParams.category).success(function(res) {

      $scope.current_category_id = $routeParams.category;
      console.log($routeParams.category);
      $scope.pageTitle = 'Posts in ' + res.data.name + ':';
      document.querySelector('title').innerHTML = 'Category: ' + res.data.name + ' | AngularJS Demo Theme';

      $http.get("~/wp/v2/posts/?filter[category_name]=" + res.data.name).success(function(res) {
        $scope.posts = res.data;
        console.log($scope.posts);
      })

    })
  }
]);

The view for the categories is the following:

<div class="list">
  <a ng-repeat="category in categories" href="#/category/{{category.id}}" class="item item-thumbnail-left">
    <h2>{{category.name}}</h2>
  </a>
</div>

And when I click on the category the URL becomes: http://localhost:8100/#/category/12, but the $stateparams.category in ~/wp/v2/terms/category/" + $routeParams.category is undefined therefor the http request cannot go through.

I cannot figure it out what am I missing ?