Oracle: Partition table by month

My solution(months in german):

PARTITION BY LIST ((to_char(GEBURTSDATUM, 'Month'))) 
(  
  PARTITION p1 VALUES('JANUAR'),  
  PARTITION p2 VALUES('Februar'), 
  PARTITION p3 VALUES('MÄRZ'),  
  PARTITION p4 VALUES('APRIL'), 
  PARTITION p5 VALUES('MAI'),  
  PARTITION p6 VALUES('JUNI'), 
  PARTITION p7 VALUES('JULI'), 
  PARTITION p8 VALUES('AUGUST'),
  PARTITION p9 VALUES('SEPTEMBER'),
  PARTITION p10 VALUES('OKTOBER'), 
  PARTITION p11 VALUES('NOVEMBER'),
  PARTITION p12 VALUES('DEZEMBER') 
);  

This doesn't work because of the to_char function.