Straddle minimum raise?

[Chipr777]

In NL, lets say a 1-3 game, if someone straddles for $6 is the minimum raise to $9 or $12? I know this has been discussed at some point in the past but I can't find the thread. My opinion is that it's $12 because the straddle is like a 3rd blind with an option and a raise has to double the blind.