Workbench Transform Left() Error

I have a field called "period" that is currently in text format and every cell has exactly six characters in this format '201701'.

I am trying to convert that to "2017-01-01" so that I can follow it up with a date transform.

Problem is, I can't even get the LEFT function to work and it is beginning to drive me nuts. I keep getting an error when I input LEFT(Period,4) - with the goal of returning "2017".

I've played with it repeatedly and keep getting the "index and length must be a location within the string error".

How is the left 4 outside of the string??