sqlite query with strftime always returns None

I have the following function:

for i in range(1,13):
        q_totes_1="""SELECT sum(kaxia) FROM es_data WHERE strftime('%%m',es_date)='%s' AND es_orig=1"""%(str(i))
        self.cur.execute(q_totes_1)
        m_totes_1=self.cur.fetchone()[0]
        print q_totes_1
        if m_totes_1 is None:
            m_totes_1=0.0

It always returns None while I know that I should have another result. From the print q_totes_1 I get the query which I execute straightly on sqlite and I get the desired result. All the imports are correct as I already have used them successfully in other functions of the same class.

I tried running a similar query without the strftime('%%m',es_date)='%s' portion and it run correctly.

Can somebody give me a hint of what I'm missing?