trouble with imdecode: always returns none. Why?

edit

I wrote streaming via UDP in python applying compression with send and rec as below

send.py

        import socket
        import numpy as np
        import cv2 as cv
        import sys
        addr = ("127.0.0.1", 65534)
        buf = 512
        width = 640
        height = 480
        cap = cv.VideoCapture('test.mp4')
        cap.set(3, width)
        cap.set(4, height)
            code = 'start'
            code = ('start' + (buf - len(code)) * 'a').encode('utf-8')
            np.set_printoptions(threshold=sys.maxsize)
            lenght=bytearray(3)
            encode_param = [int(cv.IMWRITE_JPEG_QUALITY), 90]
            s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
            frame=np.array([[[255, 255, 216],  # i use a small array instead of frame from test video to debug
              [255, 255, 215],
              [255, 255, 210],
              [255, 255, 210],
              [255, 255 ,209],
              [255, 255, 209],
              [255, 255 ,213],
              [255, 255, 213],
              [255, 255, 208],
              [255, 255 ,208]]])
            print('original')
            print(frame) 
            result, frame = cv.imencode('.jpg', frame, encode_param)
            print('frame after encode')
            print(frame)
            print('length')
            print(len(frame))             
            frame2 = cv.imdecode(frame, cv.IMREAD_COLOR)#
            print('frame2 test decode before sending')
            print(frame2)
            lenght[0]=len(frame)//(256**2)
            lenght[1]=((len(frame)%(256**2))//256)
            lenght[2]=((len(frame)%(256**2))%256)
            data=lenght+bytearray(frame) 
            for i in range(0, len(data), buf):
             s.sendto(data[i:i+buf], addr)
            cv.imshow('send', frame2)

rec.py

import socket
    import numpy as np
    import cv2 as cv
    import sys
    np.set_printoptions(threshold=sys.maxsize)
    addr = ("127.0.0.1", 65534)
    buf = 512
        width = 640
        height = 480
        code = b'start'
        num_of_chunks = width * height * 3 / buf
        data = b""
        if __name__ == '__main__':
            s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
            s.bind(addr)
            while True:
                while len(data)<3:
                 data+= s.recvfrom(buf)[0]

                length=data[0]*(256**2)+data[1]*256+data[2]
                data = data[3:]
                #print("msg_size: {}".format(length))
                while len(data) < length:
                    data+= s.recvfrom(buf)[0]

                frame_data = data[:length]
                data = data[length:]  
                frame=np.zeros((len(frame_data),1),dtype=int)
                for j in range(len(frame_data)):
                 frame[j]=frame_data[j]
                print('frame collected before decode')
                print(frame)
                print('length')        
                print(len(frame))
                frame2 = cv.imdecode(frame, cv.IMREAD_COLOR)



                cv.imshow('recv', frame2)
                if cv.waitKey(1) & 0xFF == ord('q'):
                    break

            s.close()
            cv.destroyAllWindows()

Now is the problem, the rec.py gets frame2 after decode is None. I thought my transmission is problematic so data is lost but i check "frame collected before decode"in rec.py and "'frame after encode" printed in send.py. they are identical, it means the data sent and collected properly. any opinion why decode gives None in my case?