column name disappears after applyinh astype(str) in a column in pandas dataframe

Question

userId movieId  timestamp       
1        804    964980499
1       1210    964980499
1       2018    964980523
1       2628    964980523

I have this dataframe I want the movieId to change to string so that I can use word2vec on it -- when I apply

dataframe = dataframe["movieId"].astype(str)

I get this:

userId
1         804
1        1210
1        2018
1        2628
1        2826

How can I change the type to string keeping the column name