Using isNull() clears the query in ezSQL

I am using ezSQL's selecting method for extracting data. From a table
Table Structure Table structure
Sample Data Sample Data

Using isNull method in conditions where clause clears the query. While results are stored from previous query.

I've tried using query_prepared method and it returns result from last query.

Removing isNull results in normal execution.

/* First Query */
$DB->prepareOn();
$result = $DB->selecting('reason', '*', eq('id', $reason_id));
if ($result !== null) {
    return true;
}
/* Second Query */
$result = $DB->selecting(
    "site",
    '*',
    isNull('directory')
);
print_r($DB->debug());// Shows Data of First Query

Expected output is the duckduckgo row, but the actual output is FALSE. And results are stored from last query
. Debug Mode