problem in upgrade M-S to percona5.5.21

i find a problem when i do upgrade on my M-S cluster,first i upgrade slave to percona 5.5.21,then master ,when master start
the slave always broke for error

120331 10:15:16 [ERROR] Slave I/O: Got fatal error 1236 from master when reading data from binary log: ‘Binary log is not open’, Error_code: 1236

and why it can not read the binary log automatically,i have to run “stop slave;start slave;” command manually :frowning: