How do I fix Invalid Schema when appending a BigQuery table using Pandas

I am trying to append data to a table in BigQuery using Pandas and google-cloud-big query. The problem I am having is I can create a table initially using if_exists=append but when I rerun the exact same query, I get an error -

"pandas_gbq.gbq.InvalidSchema: Please verify that the structure and data types in the DataFrame match the schema of the destination table."

Here is the code I am running:

import pandas as pd
from datetime import date
import pandas_gbq
from google.cloud import bigquery

output = [['Test', 'v1', 'cost_center', '123', date(2020, 1, 31), 30.0],
          ['Test', 'v1', 'cost_center', '345', date(2020, 1, 31), 72.0]]

headers = ['scenario', 'version', 'entity', 'account', 'period', 'amount']
df_output = pd.DataFrame(output, columns=headers)

dataset_table = 'my_dataset'
project_id ='my_project_id'
table_schema = [{'name':'scenario', 'type':'string'},
                {'name':'version', 'type':'string'},
                {'name':'entity', 'type':'string'},
                {'name':'account', 'type':'string'},
                {'name':'period', 'type':'date'},
                {'name':'amount', 'type':'float'}
]
df_output.to_gbq(destination_table=dataset_table, 
                 project_id= project_id, 
                 if_exists='append',
                 table_schema=table_schema)

It looks as if this issue has been encountered before, here. But there is no answer. Other questions on SO dealt with issues where there was a type difference between the dataframe schema and the BQ table schema.

To simplify the problem further, I tried making all columns have 'type':'String', but I get the same error message.

Thank you for the assistance.

Here is a screenshot of the table schema in BQ UI after script runs once.