MYSQLI_NUM_ROWS THROWS SERVER ERROR – Plesk

Can someone please explain to me why I keep getting the following error when I use the mysqli_num_rows() function?

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator to inform of the time the error occurred and of anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Here is my php script:

   <?php

    //stores user input as variable
    $username = $_POST['username'];
    $password = $_POST['password'];

    //selects from users table
    $sql = "SELECT * FROM Members WHERE username='$username' and password='$password'";
    $results = mysqli_query($con,$sql);

    $count = mysqli_num_rows($results);
    /*//starts session and sets cookie if count is true
    if($count>=1)
    {
        session_start();
        $_SESSION['username'] = ' ';
        $_SESSION['password'] = ' ';

        header("location: index.html"); 
    }
    //returns message if user input does not match database
    else
    {
        echo "Invalid username or password";
    }*/
    ?>

I get this error at the mysqli_num_rows() function everything before it works fine. I am using Plesk on a Godaddy server by the way.