Bios 339 error in program

I have just gotten back into computercraft and now I am getting errors from somewhere in my code.

The error I get is

bios:339:blah blah:8: end expected to close while at line 3

I have been looking for 20 mins now and I cant find the error

rednet.open("top")

while true do
  event,senderid,message = os.pullEvent("rednet_message")
  if message == "mach1 0" then
    redstone.setOutput("back", true)
  end
  else if message == "mach1 1" then
    redstone.setOutput("back", false)
  end
end

Thanks in advance!

There's no space in an elseif statement, it should be just "elseif" and you don't need an end to close an if followed by an elseif.

Cheers!