Why does this snippet throw a cyclic dependency error?

Here's the snippet in question. It seems simple enough. It actually expands to a UUID, but then it throws the attached error.

snippet uuid "Random UUID" w!
`!p import uuid; snip.rv = str(uuid.uuid4())`
endsnippet

An error occured. This is either a bug in UltiSnips or a bug in a
snippet definition. If you think this is a bug, please report it to
https://bugs.launchpad.net/ultisnips/+filebug.

Following is the full stack trace:
Traceback (most recent call last):
  File "/Users/jmcantrell/Code/dotfiles-vim/vim/bundle/ultisnips/plugin/UltiSnips/__init__.py", line 23, in wrapper
    return f(self, *args, **kwds)
  File "/Users/jmcantrell/Code/dotfiles-vim/vim/bundle/ultisnips/plugin/UltiSnips/__init__.py", line 569, in expand_or_jump
    rv = self._try_expand()
  File "/Users/jmcantrell/Code/dotfiles-vim/vim/bundle/ultisnips/plugin/UltiSnips/__init__.py", line 873, in _try_expand
    self._do_snippet(snippet, before, after)
  File "/Users/jmcantrell/Code/dotfiles-vim/vim/bundle/ultisnips/plugin/UltiSnips/__init__.py", line 847, in _do_snippet
    si = snippet.launch(text_before, self._visual_content, None, start, end)
  File "/Users/jmcantrell/Code/dotfiles-vim/vim/bundle/ultisnips/plugin/UltiSnips/__init__.py", line 417, in launch
    last_re = self._last_re, globals = self._globals)
  File "/Users/jmcantrell/Code/dotfiles-vim/vim/bundle/ultisnips/plugin/UltiSnips/text_objects/_snippet_instance.py", line 34, in __init__
    self.update_textobjects()
  File "/Users/jmcantrell/Code/dotfiles-vim/vim/bundle/ultisnips/plugin/UltiSnips/text_objects/_snippet_instance.py", line 74, in update_textobjects
    raise RuntimeError("Cyclic dependency in Snippet definition!")
RuntimeError: Cyclic dependency in Snippet definition!