merge of birthdate and birthyear when birthdate not available

gistfile1.txt

PREFIX dbo: <http://dbpedia.org/ontology/>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>

SELECT DISTINCT ?person ?birthdate
WHERE { 
	?person rdf:type dbo:Philosopher .

	{
		?person dbo:birthDate ?birthdate .
		FILTER( datatype(?birthdate) !=  xsd:gMonthDay ).
	}
	UNION {
		?person dbo:birthYear ?birthdate .

		OPTIONAL {
			?person dbo:birthDate ?b .
			FILTER( datatype(?b) !=  xsd:gMonthDay ).
		}

		FILTER( !bound(?b) )
	}
}
ORDER BY ?birthdate

if someone knows why the following query does not work, I'll be happy to think that SPARQL is simple.

PREFIX dbo: <http://dbpedia.org/ontology/>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>

SELECT DISTINCT ?person ?birthdate
WHERE { 
    ?person rdf:type dbo:Philosopher .

    OPTIONAL { ?person dbo:birthDate ?birthdate . }
    OPTIONAL { ?person dbo:birthYear ?birthdate . }
}
ORDER BY ?birthdate