Key not found when using game.set_property

Hi, i want to load a sgf, and update property like this
```
sgf_filepath = "abc.sgf"
game = sgf.load(sgf_filepath)
game.set_property("PB", "Arthur Wesley")
```

error occurs
```
Traceback (most recent call last):
  File "/Users/amoha/scripts/./update_sgf.py", line 22, in <module>
    game.set_property("PB", "Arthur Wesley")
IndexError: unordered_map::at: key not found
```

thanks