MergeError: No common columns to perform merge on
I'm trying to get through the MovieLens 1M Data Set, but am having an issue with the merge step. I checked all of my dataframes and confirmed that I successfully imported the data from the tables, even though the book/git don't have appropriately marked links. On page 28, you are supposed to merge ratings with users, then merge the result with movies. My code looks like this:

data = pd.merge(pd.merge(ratings, users), movies)

the output that results looks like this:

---

MergeError                                Traceback (most recent call last)
<ipython-input-135-65bf0f0e3564> in <module>()
      4 # pandas infers which columns to use as the join keys based on overlapping names
      5 
----> 6 data = pd.merge(pd.merge(ratings, users), movies)

//anaconda/lib/python2.7/site-packages/pandas/tools/merge.pyc in merge(left, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy)
     37                          right_on=right_on, left_index=left_index,
     38                          right_index=right_index, sort=sort, suffixes=suffixes,
---> 39                          copy=copy)
     40     return op.get_result()
     41 if **debug**:

//anaconda/lib/python2.7/site-packages/pandas/tools/merge.pyc in **init**(self, left, right, how, on, left_on, right_on, axis, left_index, right_index, sort, suffixes, copy)
    181         (self.left_join_keys,
    182          self.right_join_keys,
--> 183          self.join_names) = self._get_merge_keys()
    184 
    185     def get_result(self):

//anaconda/lib/python2.7/site-packages/pandas/tools/merge.pyc in _get_merge_keys(self)
    299         left_keys, right_keys
    300         """
--> 301         self._validate_specification()
    302 
    303         left_keys = []

//anaconda/lib/python2.7/site-packages/pandas/tools/merge.pyc in _validate_specification(self)
    398                     self.right.columns)
    399                 if len(common_cols) == 0:
--> 400                     raise MergeError('No common columns to perform merge on')
    401                 self.left_on = self.right_on = common_cols
    402         elif self.on is not None:

MergeError: No common columns to perform merge on
