Python Linear Regression Error

I have two arrays with the following values:

>>> x = [24.0, 13.0, 12.0, 22.0, 21.0, 10.0, 9.0, 12.0, 7.0, 14.0, 18.0,
...      1.0, 18.0, 15.0, 13.0, 13.0, 12.0, 19.0, 13.0]

>>> y = [10.0, 9.0, 22.0, 7.0, 4.0, 7.0, 56.0, 5.0, 24.0, 25.0, 11.0, 2.0,
...      9.0, 1.0, 9.0, 12.0, 9.0, 4.0, 2.0]

I used the scipy library to calculate r-squared:

>>> from scipy.interpolate import polyfit
>>> p1 = polyfit(x, y, 1)

When I run the code below:

>>> yfit = p1[0] * x + p1[1]
>>> yfit
array([], dtype=float64)

The yfit array is empty. I don't understand why.