Do not rely on rowCount() for SELECT statement

Which version of ThemeKey are you using?
Neither 2.x nor 3.x are using that code (anymore). The current code is

function themekey_taxonomy_nid2tid($nid) {
  $tids = array();

  $query = db_select('taxonomy_index', 't');
  $query->addField('t', 'tid');
  $query->condition('t.nid', $nid);
  $result = $query->execute();

  if ($result->rowCount()) {
    foreach ($result as $item) {
      $tids[] = $item->tid;
    }
  }

  return count($tids) ? $tids : NULL;
}

OK, you're right.

rowCount() is used four times that way. We need to fix all all of them.

I think the patch is easier than proposed because the real count is not needed.
The error has been introduced by the port from D6 to D7. In D6 the result had to be checked before you iterate over it. In D7 such a check is not required at all and could therefor be skipped completely.

The other occurrences could be fixed by using db_merge().

Can you test this patch?

I committed it to 7.x-2.x now. Thanks for your help.