Root canal and left without a temporary crown?

I got a root canal today and they said I was finished and could leave but they didn't put a temporary crown on. They told me to come back in 3 weeks for the permanent crown. I asked about the temp one and they said that my tooth was so small that he temp one would just fall off. If that's the case how is the permanent one going to stay on? Is it safe for me to go 3 weeks with just my root canaled tooth and no crown?