ABC News’ Michael Falcone reports:

CONCORD, N.H. — Presidential candidate Herman Cain, whose signature 9-9-9 tax plan would include a 9 percent national sales tax, found himself on the receiving end of an economics lesson from a New Hampshire voter today.

“I can tell you in New Hampshire people don’t like sales tax,” a diner eating lunch in a Concord restaurant told the former Godfather’s Pizza CEO.

“Don’t confuse the New Hampshire sales tax with this because you’re going to have to pay that anyway,” Cain said.

But the man quickly corrected the presidential candidate: “New Hampshire doesn’t have a sales tax.”

“Don’t mix the two,” Cain insisted. “Some people say, well we don’t want a sales tax. It’s not a state sales tax.”

The man informed Cain he was a supporter of a flat tax.

“People get nervous because of the sales tax piece,” Cain said. “But the way I respond to that is, are you happy with the current tax code? Do we know what’s in it? At least we know what’s in this.”

The conversation came during an unscheduled stop at the Ninety Nine Restaurant here. Campaign aides chose the venue for numerical reasons — the menu features “9 Real Size Entrees for Only $9.99.”

But Cain, who said he would be making more frequent visits to this important early primary state, appeared unaware that New Hampshire does not have a sales tax even though at a news conference earlier in the day a reporter who asked Cain about the 9-9-9 plan told the candidate, “New Hampshire, as you know, doesn’t have a sales tax.”

“Don’t make that mistake,” Cain told the reporter. “The state taxes aren’t in this at all. You’re going to pay the state taxes no matter what structure we have. You’re going to pay the state taxes under the old structure or you’re not going to pay state taxes, whatever the case may be, with or without it.”

“That only clouds the conversation,” he added.