[Python] NumPy, RuntimeWarning: invalid value encountered in power

I am working with Python 3.6.
I am really confused, why this happened ?

In [1]: import numpy as np

In [2]: a = np.array(-1)

In [3]: a
Out[3]: array(-1)

In [4]: a ** (1/3)
/Users/wonderful/anaconda/bin/ipython:1: RuntimeWarning: invalid value encountered in power
#!/Users/wonderful/anaconda/bin/python
Out[4]: nan

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