title: TypeError: unhashable type: 'numpy.ndarray' when merging pandas datframes from BigQuery
body: <p>I created a BigQuery dataset from the open data Github data on BigQuery.</p>
  
  <p>I am now bringing that data (two different tables) into pandas as dataframes, and it works fine. When I call 'type' on my join column, they both come up as pd.Series.</p>
  
  <p>Here's my script:</p>
  
  <pre class="lang-py prettyprint-override"><code>from google.cloud import bigquery
  from google.oauth2 import service_account
  from google.cloud import bigquery_storage_v1beta1
  from sklearn.model_selection import train_test_split
  import pandas as pd
  
  credentials = service_account.Credentials.from_service_account_file(
      'XXXXXXXXX.json')
  project_id = 'XXXXXX'
  client = bigquery.Client(credentials= credentials, project=project_id)
  
  bqstorageclient = bigquery_storage_v1beta1.BigQueryStorageClient(
      credentials=credentials
  )
  
  
  language_query = """
    SELECT repo_name, Language, Bytes,
    CASE 
    when LOWER(Language) NOT IN ('javascript', 'python', 'ruby', 'java'
                          , 'php', 'c++', 'css', 'c#', 'go'
                          , 'c', 'typescript', 'shell', 'swift'
                          , 'scala', 'objective-c') then 'Other'
    else Language END AS language_category
    FROM github_project.langauges
  
    LIMIT 1000"""
  
  language_query_dataframe = (
      client.query(language_query)
      .result()
      .to_dataframe(bqstorage_client=bqstorageclient)
  )
  
  commit_query = """
      SELECT Commit, Author, DateSeconds, repo_name
      FROM github_project.commits
      LIMIT 1000
      """
  
  commit_query_dataframe = (
      client.query(commit_query)
      .result()
      .to_dataframe(bqstorage_client=bqstorageclient)
  )
  
  merged_data = pd.merge(commit_query_dataframe, language_query_dataframe, on='repo_name')
  </code></pre>
  
  <p>That final line,</p>
  
  <pre class="lang-py prettyprint-override"><code>merged_data = pd.merge(commit_query_dataframe, language_query_dataframe, on='repo_name')
  </code></pre>
  
  <p>throws the error:</p>
  
  <pre><code>TypeError: unhashable type: 'numpy.ndarray'
  </code></pre>
  
  <p>Where is the array coming from? I can't find an answer or understand why merging on two Series should kick that out.</p>
  
  <p>Any help would be great. Thanks in advance</p>