|I'm not quite sure why you are using the entire sheet as your lookup_array, but here's why you're getting a #REF error:|
When you auto-fill down a formula it increments all relative references, including 1:65536.
I'm going to assume that you are using Excel 2003 or earlier since there are only 65536 rows available. When Excel tries to increment that reference to 2:65537, it returns a #REF error.
If you try that same operation in 2007 or beyond, it will work.
To fix it 2003 use Absolute references:
But again, why are you using the entire sheet as the lookup_array?