diff --git a/addons/account_edi_ubl_cii/models/account_edi_xml_cii_facturx.py b/addons/account_edi_ubl_cii/models/account_edi_xml_cii_facturx.py
index 61899e0c3b6..ca9a3d988d6 100644
--- a/addons/account_edi_ubl_cii/models/account_edi_xml_cii_facturx.py
+++ b/addons/account_edi_ubl_cii/models/account_edi_xml_cii_facturx.py
@@ -236,8 +236,7 @@ class AccountEdiXmlCII(models.AbstractModel):
         vals = self._export_invoice_vals(invoice)
         errors = [constraint for constraint in self._export_invoice_constraints(invoice, vals).values() if constraint]
         xml_content = self.env['ir.qweb']._render('account_edi_ubl_cii.account_invoice_facturx_export_22', vals)
-        xml_content = b"<?xml version='1.0' encoding='UTF-8'?>\n" + etree.tostring(cleanup_xml_node(xml_content))
-        return xml_content, set(errors)
+        return etree.tostring(cleanup_xml_node(xml_content), xml_declaration=True, encoding='UTF-8'), set(errors)
 
     # -------------------------------------------------------------------------
     # IMPORT
diff --git a/addons/account_edi_ubl_cii/models/account_edi_xml_ubl_20.py b/addons/account_edi_ubl_cii/models/account_edi_xml_ubl_20.py
index 164a34598fc..23104de23ca 100644
--- a/addons/account_edi_ubl_cii/models/account_edi_xml_ubl_20.py
+++ b/addons/account_edi_ubl_cii/models/account_edi_xml_ubl_20.py
@@ -478,8 +478,7 @@ class AccountEdiXmlUBL20(models.AbstractModel):
         vals = self._export_invoice_vals(invoice)
         errors = [constraint for constraint in self._export_invoice_constraints(invoice, vals).values() if constraint]
         xml_content = self.env['ir.qweb']._render(vals['main_template'], vals)
-        xml_content = b"<?xml version='1.0' encoding='UTF-8'?>\n" + etree.tostring(cleanup_xml_node(xml_content))
-        return xml_content, set(errors)
+        return etree.tostring(cleanup_xml_node(xml_content), xml_declaration=True, encoding='UTF-8'), set(errors)
 
     # -------------------------------------------------------------------------
     # IMPORT
diff --git a/addons/account_edi_ubl_cii/models/account_move_send.py b/addons/account_edi_ubl_cii/models/account_move_send.py
index f967b3f5080..2fedd367d7c 100644
--- a/addons/account_edi_ubl_cii/models/account_move_send.py
+++ b/addons/account_edi_ubl_cii/models/account_move_send.py
@@ -216,8 +216,9 @@ class AccountMoveSend(models.Model):
 
         anchor_index = tree.index(anchor_elements[0])
         tree.insert(anchor_index, etree.fromstring(to_inject))
-        invoice_data['ubl_cii_xml_attachment_values']['raw'] = b"<?xml version='1.0' encoding='UTF-8'?>\n" \
-            + etree.tostring(cleanup_xml_node(tree))
+        invoice_data['ubl_cii_xml_attachment_values']['raw'] = etree.tostring(
+            cleanup_xml_node(tree), xml_declaration=True, encoding='UTF-8'
+        )
 
     def _link_invoice_documents(self, invoice, invoice_data):
         # EXTENDS 'account'