Catchable fatal error: object of class objectCourse could not be converted to string in /objectCourse.php on line 7

objectCourse.php

class objectCourse {
	public $id;
	public $subjectID;
	public function loadSubjectID($mysqli) {
	    	$stmt = $mysqli->prepare("SELECT `Subject ID` FROM `Courses` WHERE `ID` = ?");
	    	$stmt->bind_param('s',$this->id);
	    	$stmt->execute();
	    	$stmt->bind_result($subjectID);
	    	$found = false;
	    	while($stmt->fetch()) {
	    		$found = true;
	    		$this->subjectID = $subjectID;
	    	}
	    	$stmt->close();
	        return $found;
	}
}