KeyError: 'Entity 0 does not exist in transactions'

An error of type KeyError arises when the following code is executed.

- Code:

feature_matrix_customers, feature_defs = ft.dfs(entities=es, target_entity="customers")

- Error:

KeyError                                  Traceback (most recent call last) <ipython-input-13-5a548e34c7ce> in <module>()
      1 start = time.time()
      2 feature_matrix_customers, feature_defs = ft.dfs(entities=es,
----> 3                                                target_entity="customers")
      4 stop = time.time()
      5 print("Time: {0} s.".format(stop-start))

~\AppData\Local\Continuum\anaconda2\envs\myenv\lib\site-packages\featuretools\synthesis\dfs.py in dfs(entities, relationships, entityset, target_entity, cutoff_time, instance_ids, agg_primitives, trans_primitives, allowed_paths, max_depth, ignore_entities, ignore_variables, seed_features, drop_contains, drop_exact, where_primitives, max_features, cutoff_time_in_index, save_progress, features_only, training_window, approximate, chunk_size, n_jobs, dask_kwargs, verbose)
    167     '''
    168     if not isinstance(entityset, EntitySet):
--> 169         entityset = EntitySet("dfs", entities, relationships)
    170 
    171     dfs_object = DeepFeatureSynthesis(target_entity, entityset,

~\AppData\Local\Continuum\anaconda2\envs\myenv\lib\site-packages\featuretools\entityset\entityset.py in __init__(self, id, entities, relationships)
    105         entities = entities or {}
    106         relationships = relationships or []
--> 107         for entity in entities:
    108             df = entities[entity][0]
    109             index_column = entities[entity][1]

~\AppData\Local\Continuum\anaconda2\envs\myenv\lib\site-packages\featuretools\entityset\entityset.py in __getitem__(self, entity_id)
    165         if entity_id in self.entity_dict:
    166             return self.entity_dict[entity_id]
--> 167         raise KeyError('Entity %s does not exist in %s' % (entity_id, self.id))
    168 
    169     @property

KeyError: 'Entity 0 does not exist in transactions'