diff --git a/htdocs/core/lib/bank.lib.php b/htdocs/core/lib/bank.lib.php
index 5e1e300520c..e361d8fa9f6 100644
--- a/htdocs/core/lib/bank.lib.php
+++ b/htdocs/core/lib/bank.lib.php
@@ -356,7 +356,7 @@ function checkES($IentOfi, $InumCta)
     $sum = 0;
 
     for ($i = 0; $i < 11; $i++) {
-        $sum += $values[$i] * substr($InumCta, $i, 1);
+        $sum += $values[$i] * (int)substr($InumCta, $i, 1);//int to cast result of substr to a number
     }
 
     $key = 11 - $sum % 11;