Selecting contacts on page >1 of a Group

This problem is not specific to any particular group and has been replicated across numerous groups.

It has been replicated on the 4.2 demo site and a clean install of 4.2.6.

If you select some or all of the contacts on the first page of a group (50 contacts), then select ‘Add Contacts to Group’, it operates correctly and proceeds to the next stage

If you go to the second page of contacts and repeat the process, the dialogue box appears and you can’t proceed (ie it doesn’t recognise that contacts are selected) - the modal error box says:

"Please select one or more contacts for this action.

To use the entire set of search results, click the 'all records' radio button."

The AJAX interface has been confirmed to be firing a POST request to /civicrm/ajax/markSelection with the contents below every time a check box is selected.

I have tracked this issue as far back as the countSelection() function in templates/CRM/Contact/Form - this function is returning 0 when it should be returning the number of selected rows.