wp-prepare function error "Too few arguments" - wordpress

Question

In my Wordpress plugin I have code snippets that look like this:
$total = $wpdb->get_var($wpdb->prepare(
"
SELECT SUM(Amount)
FROM $table_name
WHERE Account = $user_id AND Timestamp > {$balance['Timestamp']}
",NULL
));
It was working very well for years, but after I recently updated Wordpress to 5.0 I get many errors like this:
Fatal error: Uncaught ArgumentCountError: Too few arguments to function wpdb::prepare(), 1 passed in (...)/pluginfile.php on line 753 and exactly 2 expected in /wp-includes/wp-db.php:1222
Through my research I found that I need to use %s and %d in the wp prepare function but I didn't figure out how to apply it properly to the code above.

Answer 1

We use the 'prepare' method to make sure we're not sending an illegal operation or any illegal characters.
Try modifying your code to the following:
$total = $wpdb->get_var($wpdb->prepare(
"
SELECT SUM(Amount)
FROM $table_name
WHERE Account = %d AND Timestamp > %d
",
$user_id, $balance['Timestamp']
));
Both User ID and a timestamp are integers (whole numbers) so we would use %d.
Possible format values: %s as string; %d as integer (whole number); and %f as float.

Answer 2

For anybody that might run into a similar problem: with the help of the comment above by #entreprenerds I was able to fix the code as follows:
$total = $wpdb->get_var($wpdb->prepare(
"
SELECT SUM(Amount)
FROM $table_name
WHERE Account = %d AND Timestamp > %d
",$user_id, $balance['Timestamp']
));
Thanks!