BeautifulSoup: AttributeError: 'NavigableString' object has no attribute 'name'

Do you know why the first example in BeautifulSoup tutorial http://www.crummy.com/software/BeautifulSoup/documentation.html#QuickStart gives AttributeError: 'NavigableString' object has no attribute 'name'? According to this answer the space characters in the HTML causes the problem. I tried with sources of a few pages and 1 worked the others gave the same error (I removed spaces). Can you explain what does "name" refer to and why this error happens? Thanks.