[Python] Pythonically add header to a csv file

I wrote a Python script merging two csv files, and now I want to add a header to the final csv. I tried following the suggestions reported here and I got the following error: expected string, float found. What is the most pythonic way to fix this?

Here is the code I am using:

import csv

with open('combined_file.csv', 'w', newline='') as outcsv:
writer = csv.DictWriter(outcsv, fieldnames = ["Date", "temperature 1", "Temperature 2"])
writer.writeheader()

with open('t1.csv', 'r', newline='') as incsv:
reader = csv.reader(incsv)
writer.writerows(row + [0.0] for row in reader)

with open('t2.csv', 'r', newline='') as incsv:
reader = csv.reader(incsv)
writer.writerows(row[:1] + [0.0] + row[1:] for row in reader)

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