converting an avi file to mp4 using ffmpeg

I am trying to convert avi file to mp4 using ffmpeg. However, when i convert, the resulting file is totally distorted. can anyone help? heres my code:

import ffmpy


ff = ffmpy.FFmpeg(inputs={'ahm_0.6_core_plan.avi': None},outputs={'output.mp4': None})

ff.run()