plotly: Can’t get polar scatter working in a subplot

I am having trouble putting a histogram and polar scatter plot in two subplots. Here's my code:

import plotly
import plotly.graph_objs as go
import numpy as np

fig = plotly.tools.make_subplots(rows=1, cols=2, specs=[[{}, {}]])

trace1 = go.Histogram(
    x=np.random.uniform(1,6,size=62),
)
fig.append_trace(trace1, 1, 1)


trace2 = go.Scatter(
    r = np.random.uniform(1,6,size=62),
    t = np.random.uniform(30,5,size=62),
    mode='markers',
)
fig.append_trace(trace2, 1, 2)

plotly.offline.plot(fig, filename='temp.html')

This only renders the histogram correctly - and doesn't plot anything for the polar scatter plot. If I comment out fig.append_trace(trace1, 1, 1), I am able to get the polar scatter plot. Any ideas what am I doing wrong?