diff --git a/l10n_es_aeat_sii_oca/models/account_move.py b/l10n_es_aeat_sii_oca/models/account_move.py
index ec0054140b3..bbd96b7c433 100644
--- a/l10n_es_aeat_sii_oca/models/account_move.py
+++ b/l10n_es_aeat_sii_oca/models/account_move.py
@@ -15,6 +15,8 @@
 import json
 import logging
 
+from unidecode import unidecode
+
 from odoo import api, fields, models
 from odoo.exceptions import UserError
 from odoo.modules.registry import Registry
@@ -679,6 +681,7 @@ def _compute_sii_description(self):
                     names = invoice.mapped("invoice_line_ids.name") or invoice.mapped(
                         "invoice_line_ids.ref"
                     )
+                    names = [unidecode(x) for x in names]  # Avoid "ugly" chars
                     description += " - ".join(filter(None, names))
             invoice.sii_description = (description or "")[:500] or "/"
 
diff --git a/l10n_es_aeat_sii_oca/models/sii_mixin.py b/l10n_es_aeat_sii_oca/models/sii_mixin.py
index 1de5969d812..abaca1a4ac5 100644
--- a/l10n_es_aeat_sii_oca/models/sii_mixin.py
+++ b/l10n_es_aeat_sii_oca/models/sii_mixin.py
@@ -5,7 +5,8 @@
 # Copyright 2011,2024 Tecnativa - Pedro M. Baeza
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
 import json
-import logging
+
+from unidecode import unidecode
 
 from odoo import api, exceptions, fields, models
 from odoo.exceptions import UserError, ValidationError
@@ -14,8 +15,6 @@
 
 from odoo.addons.l10n_es_aeat.models.aeat_mixin import round_by_keys
 
-_logger = logging.getLogger(__name__)
-
 SII_STATES = [
     ("sent_modified", "Registered in SII but last modifications not sent"),
     ("cancelled", "Cancelled"),
@@ -35,7 +34,7 @@ class SiiMixin(models.AbstractModel):
         comodel_name="res.company",
         string="Company",
     )
-    sii_description = fields.Text(
+    sii_description = fields.Char(
         string="SII computed description",
         compute="_compute_sii_description",
         default="/",
@@ -797,6 +796,10 @@ def _send_document_to_sii(self):
         for document in self.filtered(
             lambda i: i.state in self._get_valid_document_states()
         ):
+            # Filter the SII description for avoiding manual invalid inputs
+            text = unidecode(document.sii_description)
+            if text != document.sii_description:
+                document.sii_description = text
             if document.aeat_state == "not_sent":
                 tipo_comunicacion = "A0"
             else: