diff --git a/functions.php b/functions.php
index 958ff24b..cc5c5b45 100644
--- a/functions.php
+++ b/functions.php
@@ -892,12 +892,17 @@ function addToMailQueue($mysqli, $data) {
 }
 
 function calculateInvoiceBalance($mysqli, $invoice_id) {
-    $sql_invoice = mysqli_query($mysqli, "SELECT * FROM invoices WHERE invoice_id = $invoice_id");
+    $invoice_id_int = intval($invoice_id);
+    $sql_invoice = mysqli_query($mysqli, "SELECT * FROM invoices WHERE invoice_id = $invoice_id_int");
     $row = mysqli_fetch_array($sql_invoice);
     $invoice_amount = floatval($row['invoice_amount']);
-    $invoice_id = intval($row['invoice_id']);
 
-    $sql_payments = mysqli_query($mysqli, "SELECT SUM(payment_amount) AS total_payments FROM payments WHERE payment_invoice_id = $invoice_id");
+    $sql_payments = mysqli_query(
+        $mysqli,
+        "SELECT SUM(payment_amount) AS total_payments FROM payments
+        WHERE payment_invoice_id = $invoice_id
+        ");
+        
     $row = mysqli_fetch_array($sql_payments);
     $total_payments = floatval($row['total_payments']);