The doubt has to be reasonable, and it undermines your defence if you don’t answer questions. The prosecutor will ask you what you were going to do with the canister. If you decline to answer, you’ve not created reasonable doubt. If you say you were going to make whipped cream, there’ll be follow-up questions, like whether you had the equipment to do so. If you don’t, then it’s not reasonable to take your claims at face value, so there isn’t reasonable doubt.