Attempt to call nil problem

Bit new to lua, but starting to get the hang of a few bits and pieces.
Trying to make a wallmaking program for a tunnel my quarry broke into, and my turtle keeps returning nil whenever I run it.
It works well untill after the checkslot() function.
Please have a look at my pastebin and tell me what part is calling nil. http://pastebin.com/5HTRxjEh
It's still in the testing phase, so not complete yet, just don't want to continue writing until I get this issue sorted out.
Thanks :)/>

Split to new topic.

turtle.detectForward()

needs to be

turtle.detect()

PixelToast, on 29 January 2013 - 10:50 AM said:

turtle.detectForward()

needs to be

turtle.detect()

Thanks, that got me over the first problem. Now I get the "call nil" error after he places the first row of the wall and he just stands there staring at it.
Did a few tweaks, so here's the new PB: http://pastebin.com/HsM26b4Z
Gone over it a bunch of times, but I guess I can't see the forest through the trees or something. All help appreciated.

turtle.right()

needs to be

turtle.turnRight()

also, the full error would be helpful (it shows the line number)

Oh, right! Sorry.

"WallsL:39:Attempt to call nil"

Edit: Well I'll be damned. I see the forest lol. Tiny typo because I got used to replacing him outside of Lua. Thanks!