Try to use ngx.null

Author: kozak

lib/resty/acme/storage/redis.lua

@@ -91,7 +91,7 @@ function _M:add(k, v, ttl)
   end
   if err then
     return err
-  elseif ok == nil then
+  elseif ok == ngx.null then
     return "exists"
   end
 end