detected empty handle

I found some strange log recectly,my version is: Version: 7.3-3,the log is as below:

Apr 18 19:00:08 postfix pmgproxy[1125188]: detected empty handle
Apr 18 19:00:13 postfix pmgproxy[1125188]: detected empty handle
Apr 18 19:00:18 postfix pmgproxy[1125188]: detected empty handle
Apr 18 19:00:23 postfix pmgproxy[1125188]: detected empty handle
Apr 18 19:00:28 postfix pmgproxy[1125188]: detected empty handle
Apr 18 19:00:32 postfix pmg-smtp-filter[1133794]: starting database maintenance
Apr 18 19:00:32 postfix...

Read more