TypeError: cannot convert the series to

I have a dataframe (df) that looks like:

date                 A
2001-01-02      1.0022
2001-01-03      1.1033
2001-01-04      1.1496
2001-01-05      1.1033

2015-03-30    126.3700
2015-03-31    124.4300
2015-04-01    124.2500
2015-04-02    124.8900

For the entire time-series I’m trying to divide today’s value by yesterdays and log the result using the following:

df["B"] = math.log(df["A"] / df["A"].shift(1))

However I get the following error:

TypeError: cannot convert the series to <class 'float'>

How can I fix this? I’ve tried to cast as float using:

df["B"] .astype(float)

But can’t get anything to work.