Unable to merge pandas dataframe with categorical values.
master_frame = pd.merge(rounds2, companies, how='inner', on='permalink')

#### Problem description

Unable to merge pandas dataframe with categorical values. Only the dataframe headers get merged. the dataframe remains empty.

File 1 details (rounds2)
rounds2.dtypes
company_permalink           object
funding_round_permalink     object
funding_round_type          object
funding_round_code          object
funded_at                   object
raised_amount_usd          float64
permalink                   object
dtype: objec

File 2 details (companies)
companies.infer_objects().dtypes
permalink        object
name             object
homepage_url     object
category_list    object
status           object
country_code     object
state_code       object
region           object
city             object
founded_at       object
dtype: object