diff --git a/facho/facho.py b/facho/facho.py
index 6304f8d..ba9b5c3 100644
--- a/facho/facho.py
+++ b/facho/facho.py
@@ -123,8 +123,7 @@ class LXMLBuilder:
 
         for el in elem.getiterator():
             is_optional = el.get('facho_optional', 'False') == 'True'
-            if is_optional and el.getchildren() == []:
-                print(tostring(el))
+            if is_optional and el.getchildren() == [] and el.keys() == ['facho_optional']:
                 el.getparent().remove(el)
 
         return tostring(elem, **attrs).decode('utf-8')
@@ -308,8 +307,12 @@ class FachoXML:
         xpath = self._path_xpath_for(xpath)
         elem = self.get_element(xpath)
 
+        if elem is None:
+            raise ValueError("xpath %s not found" % (xpath))
+
         for k, v in attrs.items():
-            self.builder.set_attribute(elem, k, v)
+            if v is not None or str(v) != 'None':
+                self.builder.set_attribute(elem, k, str(v))
         return self
 
     def get_element_attribute(self, xpath, attribute):
diff --git a/tests/test_facho.py b/tests/test_facho.py
index 19d13d7..deea299 100644
--- a/tests/test_facho.py
+++ b/tests/test_facho.py
@@ -255,3 +255,28 @@ def test_facho_xml_placeholder_set_element_to_optional_with_append():
     xml.set_element('./B', '2')
     xml.set_element('./B', '3', append_=True)
     assert xml.tostring() == '<root><A/><B>2</B><B>3</B><C/></root>'
+
+
+def test_facho_xml_set_attributes():
+    xml = facho.FachoXML('root')
+    xml.find_or_create_element('./A')
+
+    xml.set_attributes('./A',
+                       value1 = '1',
+                       value2 = '2'
+                       )
+    assert xml.get_element_attribute('/root/A', 'value1') == '1'
+    assert xml.get_element_attribute('/root/A', 'value2') == '2'
+
+
+def test_facho_xml_set_attributes_not_set_optional():
+    xml = facho.FachoXML('root')
+    xml.find_or_create_element('./A')
+
+    xml.set_attributes('./A',
+                       value1 = None,
+                       value2 = '2'
+                       )
+    with pytest.raises(KeyError):
+        xml.get_element_attribute('/root/A', 'value1')
+    assert xml.get_element_attribute('/root/A', 'value2') == '2'
diff --git a/tests/test_nomina.py b/tests/test_nomina.py
index 1aef5ac..50d9912 100644
--- a/tests/test_nomina.py
+++ b/tests/test_nomina.py
@@ -20,3 +20,31 @@ def test_adicionar_devengado_Basico():
     xml = nomina.toFachoXML()
     assert xml.get_element_attribute('/fe:NominaIndividual/Devengados/Basico', 'DiasTrabajados') == '30'
     assert xml.get_element_attribute('/fe:NominaIndividual/Devengados/Basico', 'SueldoTrabajado') == '1000000.0'
+
+def test_adicionar_devengado_transporte():
+    nomina = fe.nomina.DIANNominaIndividual()
+
+    nomina.adicionar_devengado(fe.nomina.DevengadoTransporte(
+        auxilio_transporte = fe.nomina.Amount(2_000_000)
+    ))
+
+    xml = nomina.toFachoXML()
+    assert xml.get_element_attribute('/fe:NominaIndividual/Devengados/Transporte', 'AuxilioTransporte') == '2000000.0'
+
+def test_adicionar_devengado_transporte_muchos():
+    nomina = fe.nomina.DIANNominaIndividual()
+
+    nomina.adicionar_devengado(fe.nomina.DevengadoTransporte(
+        auxilio_transporte = fe.nomina.Amount(2_000_000)
+    ))
+
+    nomina.adicionar_devengado(fe.nomina.DevengadoTransporte(
+        auxilio_transporte = fe.nomina.Amount(3_000_000)
+    ))
+
+    xml = nomina.toFachoXML()
+    print(xml)
+    assert str(xml) == """
+<NominaIndividual xmlns:facho="http://git.disroot.org/Etrivial/facho" xmlns="http://www.dian.gov.co/contratos/facturaelectronica/v1" xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2" xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2" xmlns:cdt="urn:DocumentInformation:names:specification:ubl:colombia:schema:xsd:DocumentInformationAggregateComponents-1" xmlns:clm54217="urn:un:unece:uncefact:codelist:specification:54217:2001" xmlns:clmIANAMIMEMediaType="urn:un:unece:uncefact:codelist:specification:IANAMIMEMediaType:2003" xmlns:ext="urn:oasis:names:specification:ubl:schema:xsd:CommonExtensionComponents-2" xmlns:qdt="urn:oasis:names:specification:ubl:schema:xsd:QualifiedDatatypes-2" xmlns:sts="dian:gov:co:facturaelectronica:Structures-2-1" xmlns:udt="urn:un:unece:uncefact:data:specification:UnqualifiedDataTypesSchemaModule:2" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xades="http://uri.etsi.org/01903/v1.3.2#" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns:sig="http://www.w3.org/2000/09/xmldsig#"><Devengados><Basico/><Transporte facho_optional="True" AuxilioTransporte="2000000.0"/><Transporte facho_optional="True" AuxilioTransporte="3000000.0"/></Devengados></NominaIndividual>
+"""
+