Undefined offset: 1

Hello ...
I am facing a problem that undefined offset :1 in line 3. I can't understand that what type of error it is.
Can anyone tell me that why such error occurs in php

Undefined offset in line : 3

    foreach ($lines as $line)
    {
      list($var,$value) = explode('=', $line); //line 3
      $data[$var] = $value;
    }



Question: What is the best place to start Warhammer 40k?
Answer: I personally started with Let the Galaxy Burn and the Caiaphas Cain omnibuses. From there I went into Space Marines with the Ultramarines series followed by the Soul Drinkers.

But by far my favorite two have been the Eisenhorn series and the Caiaphas Cain novels.