Weird behavior with SQLite insert or replace

I am trying to increment the count of a row in an SQLite database if the row exists, or add a new row if it doesn't exist the way it is done in this SO post. However I'm getting some weird behavior when I try to execute this SQL proc many times in quick succession. As an example I tried running this code:

db = connect_to_db()
c = db.cursor()
for i in range(10):
    c.execute("INSERT OR REPLACE INTO subject_words (subject, word, count) VALUES ('subject', 'word', COALESCE((SELECT count + 1 FROM subject_words WHERE subject = 'subject' AND word = 'word'), 1));")
    db.commit()
db.close()

And it inserted the following into the database

sqlite> select * from subject_words;
subject|word|1
subject|word|2
subject|word|2
subject|word|2
subject|word|2
subject|word|2
subject|word|2
subject|word|2
subject|word|2
subject|word|2

Which totals to 19 entries of the word 'word' with subject 'subject'. Can anyone explain this weird behavior?