create index in dbplyr requires key length in mysql

Triying to create a table from my dataframe with some indexes on integer columns gives me an error

copy_to(connection, df, indexes=list(country, date))

Error in result_create(conn@ptr, statement, is_statement) : 
  BLOB/TEXT column 'country' used in key specification without a key length [1170]

How to specify the required key length?