startup:6: attempt to concatenate string and nil

i keep getting this error
i have searched the forums but none of them seem to fix my specific problem

startup:6: attempt to concatenate string and nil

on line 6

i have no programing knowledge this is just what i have pulled from other programs and have been messing with to see how things work.

any help is greatly appreciated


my computer logon

if not fs.exists("users") then
fs.makeDir("users")
shell.run("startup")

else
local check = fs.open("users/"..username, "r")
local pass = check.readAll()
write("username: ")
local username = read()
write("password: ")
local password = read()
if not fs.exists("users/"..username) then
print("user does not exist")
os.reboot()
else
if password == pass then
term.clear()
term.setCursorPos(1,1)
else
t = 1
while true do
print("password was incorrect")
textutils.slowPrint("System Shutting Down in"..t)
t = t+ 1
sleep(10.1)
os.reboot()
end
end
end
end

Well..

local check = fs.open("users/" .. username, "r")

Where is 'username' declared?

You're declaring it after you are trying to use it, May that be the problem? I think so :P/>

Thank you for the assistance