diff --git a/services/loopback/common/locale/en.json b/services/loopback/common/locale/en.json
index 288480f559..cc493531ce 100644
--- a/services/loopback/common/locale/en.json
+++ b/services/loopback/common/locale/en.json
@@ -7,5 +7,6 @@
 	"El método de pago seleccionado requiere que se especifique el IBAN": "El método de pago seleccionado requiere que se especifique el IBAN",
 	"Ya existe un usuario con ese nombre": "Ya existe un usuario con ese nombre",
 	"Quantity cannot be zero": "Quantity cannot be zero",
-	"can't be blank": "can't be blank"
+	"can't be blank": "can't be blank",
+	"DNI Incorrecto": "DNI Incorrecto"
 }
\ No newline at end of file
diff --git a/services/loopback/common/validations/validateTin.js b/services/loopback/common/validations/validateTin.js
index ae1c38ceed..ddc3fcf63b 100644
--- a/services/loopback/common/validations/validateTin.js
+++ b/services/loopback/common/validations/validateTin.js
@@ -37,8 +37,8 @@ module.exports = function(tin, country) {
                     let sum = (pairSum + oddSum).toString();
                     let units = parseInt(sum.charAt(sum.length - 1));
                     let control = units == 0 ? 0 : 10 - units;
-                    let index = 'JABCDEFGHI'.indexOf(lastDigit);
-                    computedDigit = index == -1 ? control.toString() : index;
+                    let isLetter = /^[A-Z]$/.test(lastDigit);
+                    computedDigit = isLetter ? 'JABCDEFGHI'.charAt(control) : control.toString();
                 } else {
                     // Foreign NIF
                     let index = 'XYZ'.indexOf(tin.charAt(0));