Strange KeyError

[minhlab]

I ran into an error with this tree from CoNLL-2012 dataset:

In [1]: import StanfordDependencies

In [2]: sd = StanfordDependencies.get_instance()

In [3]: sd.convert_trees(['(TOP (S (CC But) (PRN (S (NP (PRP you)) (VP (VBP know)))) (NP (PRP you)) (VP (VBP look) (PP (IN at) (NP (NP (DT this) (NN guy)) (PRN (S (NP (PRP you))
   ...:  (VP (VBP know)))) (VP (VP (VBG punching) (NP (DT the) (CD one) (NN guy))) (VP (VBG grabbing) (NP (DT the) (NNP AP) (NN producer)) (PRN (S (NP (PRP you)) (VP (VBP know))
   ...: ))))))) (. /.)))'])
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-3-e204c241ff5e> in <module>()
----> 1 sd.convert_trees(['(TOP (S (CC But) (PRN (S (NP (PRP you)) (VP (VBP know)))) (NP (PRP you)) (VP (VBP look) (PP (IN at) (NP (NP (DT this) (NN guy)) (PRN (S (NP (PRP you)) (VP (VBP know)))) (VP (VP (VBG punching) (NP (DT the) (CD one) (NN guy))) (VP (VBG grabbing) (NP (DT the) (NNP AP) (NN producer)) (PRN (S (NP (PRP you)) (VP (VBP know))))))))) (. /.)))'])

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/StanfordDependencies/StanfordDependencies.py in convert_trees(self, ptb_trees, representation, universal, include_punct, include_erased, **kwargs)
    114                       include_erased=include_erased)
    115         return Corpus(self.convert_tree(ptb_tree, **kwargs)
--> 116                       for ptb_tree in ptb_trees)
    117
    118     @abstractmethod

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/StanfordDependencies/StanfordDependencies.py in <genexpr>(.0)
    114                       include_erased=include_erased)
    115         return Corpus(self.convert_tree(ptb_tree, **kwargs)
--> 116                       for ptb_tree in ptb_trees)
    117
    118     @abstractmethod

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/StanfordDependencies/JPypeBackend.py in convert_tree(self, ptb_tree, representation, include_punct, include_erased, add_lemmas, universal)
    139
    140         if representation == 'basic':
--> 141             sentence.renumber()
    142         return sentence
    143     def stem(self, form, tag):

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/StanfordDependencies/CoNLL.py in renumber(self)
    109             self[:] = [token._replace(index=mapping[token.index],
    110                                       head=mapping[token.head])
--> 111                        for token in self]
    112     def as_conll(self):
    113         """Represent this Sentence as a string in CoNLL-X format.  Note

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/StanfordDependencies/CoNLL.py in <listcomp>(.0)
    109             self[:] = [token._replace(index=mapping[token.index],
    110                                       head=mapping[token.head])
--> 111                        for token in self]
    112     def as_conll(self):
    113         """Represent this Sentence as a string in CoNLL-X format.  Note

KeyError: 11