If I want to say something like "Your arguement is flawed" or "His idea is completely flawed" meaning that he has overlooked something, or there is a big contradiction that makes it impossible to work.

What word can I use? All I can think of that we use in English is "flawed" and I found "difettoso" which I can obviously see looks like "defective", which is not what I'd use in English.

There are no example sentences in the dictionary for this word so I couldn't check..