php - Why do I get this function call error on an non-object when I am calling a function on an object?

Error:

Fatal error: Call to a member function
bind_param() on a non-object in
/var/www/web55/web/pdftest/events.php
on line 76

Code:

public function countDaysWithoutEvents(){       
    $sql = "SELECT 7 - COUNT(*) AS NumDaysWithoutEvents
            FROM    
            (SELECT d.date 
                FROM cali_events e
                LEFT JOIN cali_dates d
                ON e.event_id = d.event_id
                WHERE YEARWEEK(d.date) = YEARWEEK(CURRENT_DATE())
                AND c.category_id = ?

                GROUP BY DAY(d.date)
            ) AS UniqueDates";

    $stmt = $this->link->prepare($sql);
    $stmt->bind_param('i', $this->locationID);
    $stmt->execute();

    $stmt->bind_result($count);
    $stmt->close();


    return $count;
}

$this->link->prepare($sql) creates a prepared statement for MySQLi.

Why am I getting this error?

Answer

AND c.category_id = ? - there is no table alias c in your query.

Besides that try

$stmt = $this->link->prepare($sql);
if (!$stmt) {
  throw new ErrorException($this->link->error, $this->link->errno);
}

if (!$stmt->bind_param('i', $this->locationID) || !$stmt->execute()) {
  throw new ErrorException($stmt->error, $stmt->errno);
}