Golang server "write tcp ... use of closed network connection" - heroku

Question

I am beginner at Go, I had wrote small server to testing and deploy it on heroku platform. I have /logout request, which almost works, but sometimes I see something like this:
PANIC: write tcp 172.17.110.94:36641->10.11.189.195:9951: use of closed network connection
I don't know why it happens, and why sometimes it works perfectly.
My steps:
I send 1st POST request to /token-auth with body then generate token and send as response.
At 2nd I do /logout GET request with that token, and set token to Redis store
Here is full code of my redil_cli.go
package store
import (
"github.com/garyburd/redigo/redis"
)
type RedisCli struct {
conn redis.Conn
}
var instanceRedisCli *RedisCli = nil
func Connect() (conn *RedisCli) {
if instanceRedisCli == nil {
instanceRedisCli = new(RedisCli)
var err error
//this is works!!!
instanceRedisCli.conn, err = redis.Dial("tcp", "lab.redistogo.com:9951")
if err != nil {
panic(err)
}
if _, err := instanceRedisCli.conn.Do("AUTH", "password"); err != nil {
//instanceRedisCli.conn.Close()
panic(err)
}
}
return instanceRedisCli
}
func (redisCli *RedisCli) SetValue(key, value string, expiration ...interface{}) error {
_, err := redisCli.conn.Do("SET", key, value)
if err == nil && expiration != nil {
redisCli.conn.Do("EXPIRE", key, expiration[0])
}
return err
}
func (redisCli *RedisCli) GetValue(key string) (interface{}, error) {
data, err := redisCli.conn.Do("GET", key)
if err != nil{
panic(err)
}
return data, err
}
After that my function that checks Authorization header will panic while trying to do GetValue(key string) method
func (redisCli *RedisCli) GetValue(key string) (interface{}, error) {
data, err := redisCli.conn.Do("GET", key)
if err != nil{
panic(err)
}
return data, err
}
Can anyone point me, what I doing wrong?