One term in the series 6, 12, 21, 32, 48, 66 does not fit the rule followed by the rest. Which term is wrong?

The gaps should increase by 3 each time: +6, +9, +12, +15, +18. Starting at 6 that gives 6, 12, 21, 33, 48, 66. Every listed term matches except the fourth, which is printed as 32 instead of 33. So 32 is the wrong term.