Help with new program

Topic: Program reset? I have a program I've been working on that isn't quite complete, but I've run into a problem where, after line 17, where it asks "yes/no", the program resets itself and the "CraftOS 1.5" message appears back at the top of the screen. It's an advanced computer with an advanced monitor on the right of it.
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local a = ""
local m = peripheral.wrap("right")
local p = ""
while p ~= "Maximus" do
--change Maximus to your password
   term.write("Enter password: ")
   p = read()
   sleep(2)
   print("Verifying, please wait...")
   if p ~= "Maximus" then
	  sleep(3)
	  print("Incorrect, please retry")
   end
end
sleep(3)
print("Correct, hello Maximus230x")
term.write("Initiate access sequence?(yes/no)")
a = read()
if a == yes then
   print("please look at monitor")
   m.setTextScale(5)
   m.setBackgroundColor(colors.red)
   m.setTextColor(colors.black)
   m.write("Warning")
   m.write("Nuclear testing ahead")
   m.write("enter activation code")
elseif a == no then
   print("Very well, accessing files...")
   sleep(2)
   print("You may now access terminal")
else
   print("Invalid response, rebooting")
   os.reboot()
end

At line 19,

a == yes

yes is nil. I believe you meant "yes". By the way, if you had put a sleep before reboot, you would have spot that mistake a long ago.

Thank you, forgot you needed quotes around a string