Conditional If Statement: If value contains string then set another column equal to s

[Jack_Sparrow]

I have a column 'original_title', where I have different film titles i.a. all films of Star Wars (+ the name of the episode) and Star Trek (+ the name of the episode). I want to create one column which will show me only 'star trek' (without the name of episode), 'star wars' and 'na'.

This is my code for the new column:

df['Trek_Wars'] = pd.np.where(df.original_title.str.contains("Star Wars"), "star_wars", pd.np.where(df.original_title.str.contains("Star Trek"), "star_trek"))

However, it doesn't work

ValueError Traceback (most recent call last) in () 1 df['Trek_Wars'] = pd.np.where(df.original_title.str.contains("Star Wars"), "star_wars", ----> 2 pd.np.where(df.original_title.str.contains("Star Trek"), "star_trek"))

ValueError: either both or neither of x and y should be given

What should I do?