title: Dask Groupby-Apply meta failed
body: <p>I have a groupby that is working for me without using the meta argument. It outputs what I want but I would like to add column names and get a dataframe instead of a Series as an output. </p>
  
  <p>I am this trying to run the following code:</p>
  
  <pre><code>jmin = client.persist(j1.loc[:10000])
  
  import pandas as pd
  import numpy as np
  
  def unique(d):
      return len(d.loc[:,['id']].drop_duplicates())
  
  meta=pd.DataFrame(columns=['ids_per_mac'])
  meta.ids_per_mac.astype(np.int64)
  
  uu = client.persist(jmin.groupby(jmin['mac_address']).apply(unique,meta=meta))
  </code></pre>
  
  <p>The execution fails:</p>
  
  <p><a href="https://i.stack.imgur.com/7RufV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7RufV.png" alt="enter image description here"></a></p>
  
  <p>Any idea why the this groupby apply is not working vs the version without meta?</p>
  
  <pre><code>uu = client.persist(jmin.groupby(jmin['mac_address']).apply(unique))
  </code></pre>