Trying to insert a new row to DataFrame results in "TypeError: unhashable type: 'Series'"

Question

I have a DataFrame which looks like this:

          id    user  rating  rating2
347  2897196  Delora       1        3
348  1329338  Delora       2        5
349  7065882  Delora       3        1
350  1615477  Delora       4        4
351  2316564  Delora       5        5

Now I want to add a new row to this DF. However I'll get a TypeError: unhashable type: 'Series'

df2 = {'id': movie_id, 'user': 'insert', 'rating': 0, 'rating2': 3}
df = df.append(df2, ignore_index = True)

The problem lies in the following assignment:

df_new = df.append(df2, ignore_index = True)  # works, I can print the new DF and I see the added row
df = df_new                                   # <- this breaks my code => TypeError: unhashable type: 'Series'

Can someone explain me what I'm supposed to do to fix it?

Update:

This is the function in which the error occurs:

def get_AB(df_user, df_user2):
    df_merged = df_user.copy()
    df_merged['rating2'] = 0
    for x in df_user2.iterrows():
        movie_id = x[1]['id']
        if movie_id in df_user['id'].values:
            row_id = df_user['id'] == movie_id
            df_merged.loc[row_id, 'rating2'] = x[1]['rating']
            
        if movie_id not in df_user['id'].values:
            df2 = {'id': movie_id, 'user': 'insert', 'rating': 0, 'rating2': x[1]['rating']}
            df = df_merged.append(df2, ignore_index = True)
    
    return 0

The goal is to merge two DataFrames:

# DataFrame 1
          id    user  rating 
347  2897196  Delora       1       
348  1329338  Delora       2       
349  7065882  Delora       3       
350  1615477  Delora       4       
351  2316564  Delora       5       

# DataFrame 2
          id    user  rating
347    12344  Delora       1        
348  1329338  Delora       4        

To something like this:

          id    user  rating  rating2
347  2897196  Delora       1        0
348  1329338  Delora       2        4
349  7065882  Delora       3        0
350  1615477  Delora       4        0
351  2316564  Delora       5        0
xxx    12344  Delora       0        1