'Cannot reinitialize' warning using reference to dataTable defined with no arguments

For some reason this syntax is now triggering the Cannot Reinitialize DataTable warning:

var oTable = $('#tblEmailRecipients').dataTable();

I've used this elsewhere, and it's worked fine. The only thing I can think of that's different is that it's working fine when called inside an ajax call as part of the success() function, and here it's running at the top of $(document).ready() i.e. in query's top-level (global, sort of) namespace.

Does using the $().dataTable() call to get a reference to a table work only when it's inside functions? or is there some other reason this works in some places and not in others? I'm using dataTables 1.9.4.