What is error code -18???

Hi there

So I was updating my iPhone 5s and it got disconnected from the charging port. So this morning it said to connect to iTunes in recovery mode. So I did and it said it had to be restored, but when I went through the steps it kept coming up with error code 9, but then all of a sudden it said error code -18 instead. I can not find error code 18 anywhere on the internet.

Can someone please help me