send_file() is retrieving an error with a memory file

I am trying to send a file using flask:

@app.route('/genFIT', methods=["POST"])
def genFIT():

# CREATE MEMORY FILE    create an output stream
output = BytesIO()
writer = pd.ExcelWriter(output, engine='xlsxwriter')

# some code to write in the 'file'

# save
# the writer has done its job
# go back to the beginning of the stream

return send_file(output, attachment_filename="my_file" + ".xlsx", as_attachment=True)

However is returning the error:

2019-07-13 10:25:45,022: [2019-07-13 10:25:45,021] ERROR in app: Exception on /genFIT [POST]
2019-07-13 10:25:45,023: io.UnsupportedOperation: fileno

I would appreciate any guidance to overcome this issue. This code works fine at my localhost. Txs

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