Naughty birthday card signed by Princess Diana shows off her sense of humor

The late Princess Diana continues to challenge conventions, even two decades after her unfortunate death.

Back in the nineties, the Princess sent a birthday card to her accountant, Anthony Burrows, which contained a somewhat dirty joke.

Read more on our Twitter account, @amomama_usa. The card, now on auction, can be viewed below.

The outside of the card as a drawing for fairytale character Sleeping Beauty with the caption: A little prink in the hand sent Sleeping Beauty to sleep.

Innocent, right? Not so much, the inside of the card read: Only the big ones are worth staying up for!

The card is expected to sell from between $800 to $1,200 and auction, titled "Icons & Idols: Hollywood" will be open on November 16 and 17.

The Princess clearly had a wicked sense of humor, something her sons attested to in the HBO documentary about her life.

Harry said that she once told her sons they could "be as naughty as" they want, as long as they don't get caught. He added that "she was the naughtiest parent."

As many knows, Diana was also humble, but not everyone knows that she worked as a nanny for $5 an hour before she became the People's Princess.

Her employer, Mary Robertson, didn't realize at first who her nanny was. She simply needed someone to take care of her son and contacted an Au Pair agency.

Diana Spencer, then 18 years old, was the only available candidate. Her duties included taking Patrick to the park, doing the family's laundry, and picking up after the toddler.

Only when Robertson found a bank deposit receipt under her couch, did she realize her nanny was an aristocrat from one of England's oldest families.

The deposit slip was from Coutts and Company, the chosen bank for the Royal Family. Robertson was impressed and mentioned as much to Lady Diana.