Doesn't support kex algorithm 'diffie-hellman-group-exchange-sha256' even I config it in '_preferred_kex' alone

[FrankHuy]

as the title said , it doesn't support kex algorithm 'diffie-hellman-group-exchange-sha256' even I config it in '_preferred_kex' alone.

wireshark get the packet like pic under, the field kex_algorithms of SSH is [Empty], I don't know why.

[FrankHuy]

env info:
python 2.7.10
paramiko 2.7.2