Call to a member function query() on a non-object ( Help Guys! )

I am trying to write a function that inserts pieces of data into my database.

Here is what I have done so far:

function insert_data($name, $email, $subject, $question){

global $wpdb;

$wpdb->query( $wpdb->prepare( “
INSERT INTO $wpdb->[table_name]
( id, name, email, subject, questiom )
VALUES ( %d, %s, %s, %s, %s )”,
NULL, $name, $email, $subject, $question ) );

}

$name = “Joseph”;
$email = “[email protected]”;
$subject =”My Subject”;
$question = “How are you?”;
insert_data($name, $email, $subject, $question);