title: Pandas merge giving error "Buffer has wrong number of dimensions (expected 1, got 2)"
body: <p>I am trying to do a pandas merge and get the above error from the title when I try to run it. I am using 3 columns to match on whereas just before I do  similar merge on only 2 columns and it works fine.</p>
  <pre><code>df = pd.merge(df, c, how=&quot;left&quot;,
          left_on=[&quot;section_term_ps_id&quot;, &quot;section_school_id&quot;, &quot;state&quot;],
          right_on=[&quot;term_ps_id&quot;, &quot;term_school_id&quot;, &quot;state&quot;])
  </code></pre>
  <p>columns for the two dataframes</p>
  <p>df:</p>
  <pre><code>Index([u'section_ps_id', u'section_school_id', u'section_course_number', u'secti
  on_term_ps_id', u'section_staff_ps_id', u'section_number', u'section_expression'
  , u'section_grade_level', u'state', u'sections_id', u'course_ps_id', u'course_sc
  hool_id', u'course_number', u'course_schd_dept', u'courses_id', u'school_ps_id',
   u'course_school_id', u'school_name', u'school_abbr', u'school_low_grade', u'sch
  ool_high_grade', u'school_alt_school_number', u'school_state', u'school_phone',
  u'school_fax', u'school_principal', u'school_principal_phone', u'school_principa
  l_email', u'school_asst_principal', u'school_asst_principal_phone', u'school_ass
  t_principal_email'], dtype='object')
  </code></pre>
  <p>c:</p>
  <pre><code>Index([u'term_ps_id', u'term_school_id', u'term_portion',
  u'term_start_date', u' term_end_date', u'term_abbreviation',
  u'term_name', u'state', u'terms_id', u'sch ool_ps_id',
  u'term_school_id', u'school_name', u'school_abbr', u'school_low_grad
  e', u'school_high_grade', u'school_alt_school_number',
  u'school_state', u'school
  _phone', u'school_fax', u'school_principal', u'school_principal_phone', u'school
  _principal_email', u'school_asst_principal', u'school_asst_principal_phone', u's chool_asst_principal_email'],
  dtype='object')
  </code></pre>
  <p>Is it possible to merge on three columns like this? Is there anything wrong from the merge call here?</p>