[SOLVED] Num_rows() returns error message

fatal error: call to a member function num_rows() on boolean

That means that $query is a TRUE/FALSE and not an object. Likely FALSE due to query failure.

I know, but how can $this->db->list_tables() cause this error?
In MySQL, I've never seen this error. There, I can check for $query->num_rows() == 0); This doesn't seem to work in MSSQL (Windows Server).