Formula works for one row does not for two

[Prometheus]

Hi

Trying to extract the core of the problem, I have found that a formula works for one row only, if another row is added, an error shows:
cannot convert the series to <class 'float'>

By the way, does not work even if the content is turned to float beforehand.

So here it is

import pandas as pd
import numpy as np

startlat=51.454234 
startlon=-2.593225

df6 = pd.DataFrame(columns=['Latitude', 'Longitude'])
df6.loc[0] = [51.3862652,  -2.3638941]
# df6.loc[1] = [51.3862652,  -2.3638941]

df6['dist1']=6371.01 * np.arccos(np.sin(np.radians(float(startlat)))*np.sin(np.radians(float(df6['Latitude']))) + np.cos(np.radians(float(startlat)))*np.cos(np.radians(float(df6['Latitude'])))*np.cos(np.radians(float(startlon)) - np.radians(float(df6['Longitude']))))
df6.head()

This works. However, when I uncomment the addition of the second row and run this all again, python returns an error: TypeError: cannot convert the series to <class 'float'>

What is the cause and how to work around that?

Thank you.