pandas.Series.unique() of a feature has 0 while pandas.Series of the same feature has no 0

Question

I have a feature LotFrontage in dataset.

print(0 in dataset['LotFrontage']) #Prints True
print(0 in dataset['LotFrontage'].unique()) #Prints False

I think 0 should be present in the unique values of LotFrontage only if its present in LotFrontage. Note that the feature has no 0 values.