failed to set keepalive: connection in dubious state

local redis = require "resty.redis"
local red = redis:new()

red:set_timeout(1000) -- 1 sec

local ok, err = red:connect("127.0.0.1", 6379)
if not ok then
ngx.say("failed to connect: ", err)
return
end

--ngx.say("connected to redis.")

local key = "testkey"
red:set(key, false)

local ok, err = red:set_keepalive(10000, 100)
if not ok then
ngx.say("failed to set keepalive: ", err)
return
end

The code bellow throw an error: "failed to set keepalive: connection in dubious state"

I know redis set support string value only, But "red:set(key, true)" will be ok, why?

Start with the supplied Lua reproduction, especially red:set(key, false) and red:set_keepalive(10000, 100); compare it with the true case. Trace how these calls affect the Redis connection and determine whether the dubious-state error is expected or a bug. Done means explaining the difference and verifying the behavior or its fix.