AD9834: output sin frequency is not exactly like set in Software

Hello everyone in forum,

I am using STM32F7 to provide the mclk(currently it is 1 MHz) and configuration via SPI to generate the sin wave from AD9834. But I found some issues, the output for 1000 Hz, 2000Hz and 10kHz are good enough, but at 500 Hz there is big deviation, actually I got 530 Hz when I configure the frequency to 500Hz. And If I configure the frequency less than 100 Hz, the output is always 43Hz, not changed. has anyone also had this kind issues or can someone help please? thank you very much for the help and hints in advance.

sincerely 

Shangping