Predict a negative number for a stock whose entire history has been positive.

Hello, I used the Prophet model to forecast Apple's stock ('AAPL') for the next 15 days. I was astonished to see that the model predicts negative figures for AAPL stock, which has never had a negative value before.
What may be the source of the issue?
Sample issue:
Code:
```
today = datetime.today().strftime('%Y-%m-%d')
start_date = '2016-01-01'
eth_df = yf.download('AAPL',start_date, today)
eth_df.reset_index(inplace=True)
df = eth_df[["Date", "Open"]]
new_names = {
"Date": "ds",
"Open": "y",
}
df.rename(columns=new_names, inplace=True)
m = Prophet(
seasonality_mode="multiplicative"
)
m.fit(df)
future = m.make_future_dataframe(periods = 15)
forecast = m.predict(future)
plot_plotly(m, forecast)
```