print_error does not allow more than one parameter ($a) in the string

Trying to put an error message using qtype_multianswer.php with 2 parameters
$string['unknownquestiontypeofsubquestion'] = 'Unknown question type: $a->type of question part # $a->sub';
with the following code
$a = new stdClass;
$a->type = 'multichoice' ;
$a->sub = "1";
echo "<p>".get_string('unknownquestiontypeofsubquestion', 'qtype_multianswer',$a)."</p>";
print_error('unknownquestiontypeofsubquestion', 'qtype_multianswer',$a);
the echo get_string is Ok but the print_error has an error from weblib. see the screenshot to be attached