完整性约束违规：1048列“建议”不能为空

Every time I try to press submit without typing anything in the suggestion box on my website, the website is supposed to deny the blank box from the SQL suggestions database and display an error, but when pressing Ok to the alert, it quickly displays an error saying 'Integrity constraint violation: 1048 Column 'Suggestion' cannot be null' in /home/u611142741/public_html/suggestions.php:41'.

Here is my PHP code:

$conn=mysql_connect("localhost", "u611142741_list", "REDACTED"); 
mysql_select_db("u611142741_sugge", $conn);

$db = new PDO("mysql:host=localhost;dbname=u611142741_sugge;","u611142741_list","REDACTED");
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

// If the form has been submitted
if (trim($_POST["suggestion543"]) == "") {
  echo "Error";
  echo '<script language="javascript">';
  echo 'alert("Invalid entry! Try Again.")';
  echo '</script>';
  echo '<script>';
  echo 'setTimeout(function(){ window.location="http://musicoro.tk/suggestion-form/" }, 500)';
  echo '</script>';
} else {
  $suggestion = $_POST['suggestion543'];
  $ip = $_SERVER['REMOTE_ADDR'];
  echo "Thank you for submitting your suggestion!";
  echo '<script>';
  echo 'setTimeout(function(){ window.location="http://musicoro.tk/suggestion-form/" }, 2000)';
  echo '</script>';
};



// Build an sql statment to add the student details
$stmt = $db->prepare('INSERT INTO suggestions

(`Suggestion`, `IP Address`) VALUES

(:suggestion, :ip)');

$stmt->bindParam(':suggestion', $suggestion, PDO::PARAM_STR);
$stmt->bindParam(':ip', $ip, PDO::PARAM_STR);
try {
        $stmt->execute();
}
catch(PDOException $e) {
        echo $e->getMessage();
}

$result = mysql_query($sql,$conn);


// close connection 
mysql_close($conn);

And this was put into my error logs:

[23000]: Integrity constraint violation: 1048 Column 'Suggestion' cannot be null' in /home/u611142741/public_html/suggestions.php:41
Stack trace:
#0 /home/u611142741/public_html/suggestions.php(41): PDOStatement->execute()
#1 {main}
  thrown in /home/u611142741/public_html/suggestions.php on line 41
[22-Nov-2014 22:59:06 UTC] PHP Fatal error:  Uncaught exception 'PDOException' with message 'SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'Suggestion' cannot be null' in /home/u611142741/public_html/suggestions.php:41
Stack trace:
#0 /home/u611142741/public_html/suggestions.php(41): PDOStatement->execute()
#1 {main}
  thrown in /home/u611142741/public_html/suggestions.php on line 41

Got any ideas?