Sharepoint javascript object model, access denied error

I'm using JavaScript object model to retrieve data from a list located a root site.

My code is as below:

var clientContext;
                    SP.SOD.executeFunc('sp.js', 'SP.ClientContext', function () {
                        clientContext = SP.ClientContext.get_current();
                    });

                    //var clientContext = SP.ClientContext.get_current();
                    if (clientContext != undefined && clientContext != null) {
                        var context = new SP.ClientContext.get_current();
                        this.web<%=SerialNum%> = clientContext.get_site().get_rootWeb();
                        var list = this.web<%=SerialNum%>.get_lists().getByTitle('<%=SubCategoryListName%>');
                        var query = '<View Scope=\'RecursiveAll\'>' +
                        '<Query>' +
                            '<Where>' +
                               '<Eq>' +
                                '<FieldRef Name=\'' + '<%=SubCategoryListParentCategoryColoumnName%>' + '\' LookupId=\'TRUE\'/>' +
                                '<Value Type=\'Lookup\'>' + chosenoption.value + '</Value>' +
                                                    '</Eq>' +
                                                '</Where>' +
                                        '</Query>' +
                                            '</View>';
                        var camlQuery = new SP.CamlQuery();
                        camlQuery.set_viewXml(query);
                        this.productcollection<%=SerialNum%> = list.getItems(camlQuery);

                        context.load(this.productcollection<%=SerialNum%>);
                        context.executeQueryAsync(Function.createDelegate(this, processProyects<%=SerialNum%>), Function.createDelegate(this, javascriptSPerror<%=SerialNum%>));

The problem is when I call this code from a page on the root site it works perfectly fine, but when I run it on a page located on a subsite I get the following error:

"Access denied, you don't have the permission to perform this action".

Any help is greatly appreciated.