oSettings.aoData[iRow] is undefined

Hello,

I use jquery dataTables version 1.9.4.
I have a dataTable with grouping and I display the row count of each group.
This is the code causing the error:

$(document).ready(function() {
$('#users-table').dataTable()
.rowGrouping({
bExpandableGrouping: true,
fnOnGroupCompleted: function(oGroup) {
var length = $('#users-table tr' + oGroup.groupItemClass).length;
$(oGroup.nGroup).find("td").find("sub").text("[" + length + "]");
}
});
});

and this is the error:
TypeError: oSettings.aoData[iRow] is undefined

Please, can you tell me the cause of the error.

thanks in advance

please accept my sincere greetings