1.6 mysql num_rows error

[eluser]Tobz[/eluser]
Hi, I've just upgraded to CI 1.6

and I'm having a new mysql error in my error logs

ERROR - 2008-02-04 11:19:00 --> Severity: Warning --> mysql_num_rows(): supplied argument is not a valid MySQL result resource D:\web\Codeigniter\1.6.0\system\database\drivers\mysql\mysql_result.php 37

The weird thing is I'm not doing any sql in the controller/function that I'm calling (there is 1 sql call in the controller but different function).
So I don't know where to start debugging as its not in my code anywhere.

Any help would be appreciated.
Thanks