Why there is an empty line begin with "*" in the result of history command on linux?

Today at work, I wanna know a history command which I am not very familiar with and I was sure that I just executed a little while ago. However, when I ran history on the same terminal window, I found nothing about the command. What made me more confused, or even annoyed was that I found an empty line begin with a "*" in the output of history. Is there someone who can tell me why? Please give some specific explanations.

 2008  make build_asdfasdfa
 2009  ls
 2010  cd asdfasdf
 2011  ls
 2012* 
 2013  exxxxxxxxxxxxxxxxxxx
 2014  ls
 2015  exxxxxxxxxxxxxxxxxx
 2016  ls
 2017  exxxxxxxxxxxxxxxxxxxxxx
 2018  ls

Solution 1:

History lines which have been modified, are shown with a * at the beginning of the command, provided that mark-modified-lines property has been turned on in ReadLine configuration.

Here what ReadLine documentation says:

mark-modified-lines
    This variable, when set to On, says to display an asterisk ('*') at the
    start of history lines which have been modified. This variable is off
    by default.