html - PHP Undefined Index

This is going to sound really stupid, but I cannot figure out why I am getting this error.

I have created a selection box, named "query_age" in my html form:

  Doesn't matter
  18 - 30
  31 - 40
  41 - 50
  51 - 60
  61 - 70
  71 - 80
  81 - 90
  Older than 90

In the corresponding php form, I have:

$query_age = $_GET['query_age'];

When I run the page, I get this error:

Notice: Undefined index: query_age in index.php on line 19

I don't understand why this is happening, and I'd love to know how to make it go away.