Think about what users could say
Before asking a question, think about what responses you can reasonably support. Don’t ask the user a question if you’re not prepared to handle their answer.
That said, don’t be afraid to ask the user a question—it doesn’t mean that you have to support every response imaginable. The way a question is phrased sets the user’s expectations for what they can say. This phrasing can range from open-ended, or wide-focus questions, to close-ended, or narrow-focus questions.
The magic and art of good conversation design is that users feel like they’re in control and that they can say anything at anytime, but in reality, the dialog directs them along pre-scripted paths.