SQL Server XML .Value query returns NULL, should return a value

< Highly mocked up SQL Server 2008R2 T-SQL >

DECLARE @x XML 
SET @x = '<goo xmlns="http://underdog.com" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
  <foo>
    <boo>banana</boo>
  </foo>
</goo>'
SELECT @x.value('(/goo/foo/boo)[1]', 'varchar(64)') AS the_amount

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I'm expecting to return a the_amount value of 'banana', but I'm always getting NULL.  What's the fix?