DataFrames. Cannot change missing values

Would you help me?
I have a DataFrame. Add a new column with missing values, but later I can not change the value.

MethodError: Cannot convert an object of type Int64 to an object of type Missing

thank you.
P.S. How to add a column to the end of a dataframe by default.

using DataFrames
df = DataFrame(a = 1:3)
df[:b] = missings(Int, 3)
df[1, :b] = 2
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