pystore: Append not working with Dask > 2.1.0

Append stopped working for me in 0.1.12

My reproducible example:

import random
import datetime as dt
import pandas as pd
import pystore as ps

#create df with random numbers and timestamp as index
n = 2
timestamps = [dt.datetime.utcnow() + dt.timedelta(seconds=x) for x in range(n)]
pb = ([{'timestamp': timestamp, 'pb': random.random()} for timestamp in timestamps])
df = pd.DataFrame.from_dict(pb).set_index('timestamp')

store = ps.store('mydatastore') 
collection = store.collection('eod')

#write to pystore
collection.write('symbol', df[:-1], overwrite=True)

#append to pystore
collection.append('symbol',  df[-1:]) 

#read 
collection.item('symbol').to_pandas()

This works in 0.1.9 Would be grateful for any hint.

Kind regards Steffen