I2C slave transmit getting first byte as ffffffff

[RChan.3]

I am using i2c ,arduino as master and stm32l073RZ as slave.When I transmit from arduino(master) to slave(st) via i2c.I am getting correct values.But when I slave transmit from st the first byte received in arduino-master is ffffffff,the other bytes are correct.What is this issue??I am generated the code by cubeide.