emptying Lines data breaks figure

[kecnry]

Describe the bug
Emptying the x/y data in a Lines instance clears the mark, as expected, but future updates fail to update on the figure

To Reproduce

import bqplot
import numpy as np

x = np.linspace(0,1,11)
y = x**2

fig = bqplot.Figure()
fig.axes = [bqplot.Axis(scale=bqplot.LinearScale(), label='x'),
            bqplot.Axis(scale=bqplot.LinearScale(), orientation='vertical', label='y')]

line = bqplot.Lines(scales={'x': fig.axes[0].scale,
                            'y': fig.axes[1].scale},
                    x=x, y=y,
                    colors=['gray'], size=12)

fig.marks = fig.marks + [line]

fig

(displays as expected)

line.x, line.y = [], []

(clears plot as expected, no errors, but without this line the next line would work as expected)

line.x, line.y = x, 2*x

(plot does not update and remains blank - although grid lines do reappear, the following error appears in the JS console)

Error setting state: invalid format: .undefinedg

Expected behavior
Line should re-appear after setting to be non-empty array.