Something was wrong when I try to read data from the DMP.

Hi,
I download the Embedded_MotionDriver_5.1 and try to use the mpu6050's DMP. Everything seems ok except that I can't read data from the fifo , I found
the code which caused this ,
int mpu_read_fifo_stream(unsigned short length, unsigned char *data,
unsigned char *more)
{
unsigned char tmp[2];
unsigned short fifo_count;
if (!st.chip_cfg.dmp_on)
return -1;
if (!st.chip_cfg.sensors)
return -1;

if (i2c_read(st.hw->addr, st.reg->fifo_count_h, 2, tmp))//the value of "tmp" is always 0, but I am sure the i2c wire works well.

Could you tell me what's wrong with it ?
Thanks!