Problem Trying to Sum SQL decimals with specific ID

I am trying to add up all the values within a decimal column where the ID is equal to a queried ID. I have tried many solutions and nothing seems to work. The current solution seems to provide an answer however it is the wrong one.

/* Sum payments made to invoice number */
$payments_table = $wpdb->prefix . "_payments";
$invoice_id = '999999999';

$result = $wpdb->query('SELECT sum(value) FROM '.$payments_table.' WHERE invoice_id = '.$invoice_id.'');
echo $result;

In the database there are currently two payments of ‘0.001’ for invoice ‘999999999’ so I would expect the sum to be ‘0.002’ however the code returns ‘1’