ssl_crl parameter exception "Broken pipe (Write failed)"

[liugh1974]

I try to configure tcp with ssl, please see the following configuration: nginx.conf

tcp {

    upstream cluster {
        server localhost:8883;
        server localhost:8884;
        check interval=3000 rise=2 fall=5 timeout=1000;
    }

    server {
        listen 8888 ssl;
        proxy_pass             cluster;

        ssl on;
        ssl_certificate                crts/nginx_server.cert.pem;
        ssl_certificate_key            crts/nginx_server.key.pem;
        ssl_client_certificate         crts/ca-chain.pem;
        ssl_crl                        certs/ca.crl.pem;

        ssl_verify_client      on;
        ssl_ciphers            HIGH:!aNULL:!MD5;
        ssl_session_cache      shared:SSL:20m;
        ssl_session_timeout    4h;
        ssl_verify_depth       2;
    }
}

When there isn't ssl_crl parameter, all are fine. but when I add ssl_crl, the client always handshake failure with exception "Broken pipe (Write failed)". I'm sure that ca.crl.pem is a correct crl file, I have validated it correctly with openssl.

Could you tell me why?