vsftpd only support tlsv1.2

Question

I have installed the vsftpd 3.0.2 server in ubuntu. And I configure the ssl support with following:

ssl_tlsv1=YES
ssl_sslv2=NO
ssl_sslv3=NO

It works fine, but now I want my ftp server only support tlsv1.2, how to configure it? I have tried as following:

ssl_tlsv1_2=YES
ssl_sslv2=NO
ssl_sslv3=NO

But it doesn't work, when I try to start the ftp server, error happens:

500 OOPS: unrecognised variable in config file: ssl_tlsv1_2

And I do not find the parameter ssl_tlsv1_2 vsftpd.conf

But I find the parameter ssl_tlsv1_2 vsftpd.conf

That confuses me very much.