Before you go, we thought you'd like these...  Chelsea Handler just gave her friend Reese Witherspoon a birthday gift she'll surely never forget.  Naturally, the comedian took to Instagram on Tuesday to with the actress a happy birthday by sharing a completely nude mirror selfie. Something tells us that Witherspoon, who turned 40 on Tuesday and performed with Keith Urban at her birthday party over the weekend, will greatly appreciate Handler's snap, in which she's wearing knee-high black socks and nothing else.  SEE ALSO: Hilarious phone call Miley Cyrus made to a DJ  The "Chelsea Does" star has made it her mission over the last year or so to become a fierce advocate for the #FreeTheNipple campaign, so it may come as a surprise to some people that Handler opted to cover her private areas in this shot.  Regardless, there's no getting around the fact that the best-selling author's bod looked incredible in the picture, which looks to have been taken in her bathroom.  See more photos of Chelsea Handler through the years:  Chelsea Handler gives Reese Witherspoon a birthday present she'll never forget  Chelsea Handler at Netflix's FYC "Orange is the New Black" Emmy Panel on Monday, August 4, 2014, in Los Angeles. (Photo by Alexandra Wyman/Invision for Netflix/AP Images)  Get on my level, Sarah Silverman! Check out our #SB49 #ad for @TMobile on their YouTube.  NEW YORK, NY - OCTOBER 09: Chelsea Handler arrives for the 'Late Show with David Letterman' at Ed Sullivan Theater on October 9, 2014 in New York City. (Photo by Donna Ward/Getty Images)  LOS ANGELES, CA - JANUARY 06: (L-R) Actress Mary McCormack, author Arianna Huffington, and comedian Chelsea Handler attend as Arianna Huffington hosts a special lunch at home for Jennifer Aniston to celebrate CAKE on January 6, 2015 in Los Angeles, California. (Photo by Ari Perilstein/Getty Images for LTLA)  LOS ANGELES, CA - JANUARY 06: (L-R) Comedian Chelsea Handler, author Arianna Huffington and actors Jennifer Aniston and Justin Theroux attend as Arianna Huffington hosts a special lunch at home for Jennifer Aniston to celebrate CAKE on January 6, 2015 in Los Angeles, California. (Photo by Ari Perilstein/Getty Images for LTLA)  Chelsea Handler arrives at the Kennedy Center for the Performing Arts for the Mark Twain Prize for American Humor on Sunday, Oct. 19, 2014, in Washington. (AP Photo/Kevin Wolf)  Chelsea Handler speaks on stage at the MTV Video Music Awards at The Forum on Sunday, Aug. 24, 2014, in Inglewood, Calif. (Photo by Matt Sayles/Invision/AP)  Comedian, actress and author Chelsea Handler poses for photos at Book Expo America, Thursday, May 30, 2013 in New York. (AP Photo/Mark Lennihan)  HOLLYWOOD, CA - JUNE 06: TV personality Chelsea Handler attends the 2014 AFI Life Achievement Award: A Tribute to Jane Fonda at the Dolby Theatre on June 5, 2014 in Hollywood, California. Tribute show airing Saturday, June 14, 2014 at 9pm ET/PT on TNT. (Photo by Kevin Mazur/WireImage)  HOLLYWOOD, CA - JUNE 06: Actress Sandra Bullock (L) and tv personality Chelsea Handler attend the 2014 AFI Life Achievement Award: A Tribute to Jane Fonda at the Dolby Theatre on June 5, 2014 in Hollywood, California. Tribute show airing Saturday, June 14, 2014 at 9pm ET/PT on TNT. (Photo by Kevin Mazur/WireImage)  HOLLYWOOD, CA - JUNE 06: TV personality Chelsea Handler (L) and actress Mary McCormack attend the 2014 AFI Life Achievement Award: A Tribute to Jane Fonda at the Dolby Theatre on June 5, 2014 in Hollywood, California. Tribute show airing Saturday, June 14, 2014 at 9pm ET/PT on TNT. (Photo by Kevin Mazur/WireImage)  HOLLYWOOD, CA - JUNE 05: TV personality Chelsea Handler attends the 2014 AFI Life Achievement Award: A Tribute to Jane Fonda at the Dolby Theatre on June 5, 2014 in Hollywood, California. Tribute show airing Saturday, June 14, 2014 at 9pm ET/PT on TNT. (Photo by Frederick M. Brown/Getty Images for AFI)  AUSTIN, TX - MARCH 30: Chelsea Handler signs copies of her new book 'Uganda Be Kidding Me' at Book People on March 30, 2014 in Austin, Texas. (Photo by Gary Miller/Getty Images)  GLENDALE, CA - MARCH 11: Gwyneth Paltrow and Chelsea Handler attend an Evening With Chelsea Handler at Alex Theatre on March 11, 2014 in Glendale, California. (Photo by JB Lacroix/WireImage)  Chelsea Handler speaks at the Ms. Foundation for Women Gloria Awards at Cipriani 42nd Street on Thursday, May 1, 2014 in New York. (Photo by Scott Roth/Invision/AP)  Chelsea Handler, Gabourey Sidibe, Gloria Steinem, Amy Schumer, and Kathy Najimy attend the Ms. Foundation for Women Gloria Awards at Cipriani 42nd Street on Thursday, May 1, 2014 in New York. (Photo by Scott Roth/Invision/AP)  FILE - In this May 2, 2013 file photo, Chelsea Handler arrives at "An Unforgettable Evening" benefiting EIF's Women's Cancer Research Fund at The Beverly Wilshire in Beverly Hills, Calif. Handler has set the date for ending her E! network comedy-talk show. "Chelsea Lately" will come to a close on Aug. 26, the network said Wednesday, May 28, 2014. (Photo by Dan Steinberg/Invision/AP, File)  Chelsea Handler arrives at the "Stand Up For Gus" Benefit at Bootsy Bellows on Wednesday, Nov. 13, 2013 in West Hollywood, Calif. (Photo by Paul A. Hebert/Invision/AP)  Chelsea Handler at An Evening with Shameless, on Tuesday, June, 4, 2013 in Los Angeles. (Photo by Eric Charbonneau/Invision for Showtime/AP Images)  EXCLUSIVE - Sandra Bullock, Chelsea Handler and President of Worldwide Marketing and International Distribution, Warner Bros. Pictures Sue Kroll at Sandra Bullock Hands and Footprints Ceremony, on Wednesday, September, 25, 2013 at the TCL Chinese Theatre in Hollywood, CA. (Photo by Eric Charbonneau/Invision for Warner Bros./AP Images)  Chelsea Handler arrives at the LA private screening of "Shameless" at the Leonard H. Goldenson Theatre on tuesday, June 4, 2013 in Los Angeles. (Photo by Richard Shotwell/Invision/AP)  Chelsea Handler arrives at "An Unforgettable Evening" benefiting EIF's Women's Cancer Research Fund at The Beverly Wilshire on Thursday, May 2, 2013, in Beverly Hills, Calif. (Photo by Dan Steinberg/Invision/AP)  Strange: Ellen DeGeneres and Chelsea Handler Have an Awkward Shower Fight http://t.co/OstTX2IAAQ via @mashable http://t.co/HIO0PQl9Fl  NEW YORK, NY - OCTOBER 09: TV personality Chelsea Handler arrives at 'Late Show with David Letterman' at Ed Sullivan Theater on October 9, 2014 in New York City. (Photo by Taylor Hill/FilmMagic)  Chelsea Handler hosts the Women in Film Crystal Lucy Awards on Friday June 12, 2009, in Los Angeles. (AP Photo/Matt Sayles)  Comedian Chelsea Handler arrives at the Chelsea Handler "Chelsea Chelsea Bang Bang" book launch party in Beverly Hills, Calif. on Wednesday, March 17, 2010. (AP Photo/Dan Steinberg)  Chelsea Handler speaks at the Ms. Foundation for Women Gloria Awards at Cipriani 42nd Street on Thursday, May 1, 2014 in New York. (Photo by Scott Roth/Invision/AP)  Host Chelsea Handler is seen backstage at the MTV Video Music Awards on Sunday, Sept. 12, 2010 in Los Angeles. (AP Photo/Chris Pizzello)  Comedian Chelsea Handler inside at the Chelsea Handler "Chelsea Chelsea Bang Bang" book launch party in Beverly Hills, Calif. on Wednesday, March 17, 2010. (AP Photo/Dan Steinberg)  Chelsea Handler, a cast member in "This Means War," poses at the premiere of the film in Los Angeles, Wednesday, Feb. 8, 2012. The film opens in theaters on Friday, Feb. 17. (AP Photo/Chris Pizzello)  Chelsea Handler from the shows "Chelsea Lately" and "After Lately" attends an E! Network upfront event at Gotham Hall on Monday, April 30, 2012 in New York. (AP Photo/Evan Agostini)  Chelsea Handler from the shows "Chelsea Lately" and "After Lately" attends an E! Network upfront event at Gotham Hall on Monday, April 30, 2012 in New York. (AP Photo/Evan Agostini)  Comedian Chelsea Handler, right, and boyfriend hotelier Andre Balazs attend the TIME 100 gala, celebrating the 100 most influential people in the world, at the Frederick P. Rose Hall on Tuesday, April 24, 2012 in New York. (AP Photo/Evan Agostini)  TV host Chelsea Handler attends the "Stand Up to Cancer" event at the Shrine Auditorium on Friday, Sept. 7, 2012 in Los Angeles. The initiative aimed to raise funds to accelerate innovative cancer research by bringing new therapies to patients quickly. (Photo by John Shearer/Invision/AP)  Chelsea Handler attends the premiere of "Movie 43" at the TCL Chinese Theatre on Wednesday, Jan. 23, 2013, in Los Angeles. (Photo by Matt Sayles/Invision/AP)  Talk show host Chelsea Handler attends the E! Network 2013 Upfront at the Manhattan Center on Monday April 22, 2013 in New York. (Photo by Evan Agostini/Invision/AP)  Sandra Bullock, left, poses with Chelsea Handler at Bullock's hand and footprint ceremony at the TCL Chinese Theatre on Wednesday, Sept. 25, 2013 in Los Angeles. (Photo by Jordan Strauss/Invision/AP)  NEW YORK, NY - OCTOBER 06: (Exclusive Coverage) Dave Grohl and Chelsea Handler at SiriusXM Studio on October 6, 2014 in New York City. (Photo by Kevin Mazur/Getty Images for SiriusXM)  HOLLYWOOD, CA - JUNE 05: TV personality Chelsea Handler attends the 2014 AFI Life Achievement Award: A Tribute to Jane Fonda at the Dolby Theatre on June 5, 2014 in Hollywood, California. Tribute show airing Saturday, June 14, 2014 at 9pm ET/PT on TNT. (Photo by Frederick M. Brown/Getty Images for AFI)  RIDGEWOOD, NJ - MARCH 08: Chelsea Handler promotes 'Uganda Be Kidding Me!' at Bookends Bookstore on March 8, 2014 in Ridgewood, New Jersey. (Photo by Jim Spellman/WireImage)  Chelsea Handler ends her cable run on a high note - in the shower with Ellen! http://t.co/yafutBcDu1 via @decider http://t.co/L7LipNKbkg  More from AOL.com: Margot Robbie set to play figure skater Tonya Harding in 'I, Tonya' film Stars react to devastating Brussels terrorist attacks Paul Qui, 'Top Chef' winner, arrested for allegedly assaulting girlfriend
Chelsea Handler took to Instagram to wish her pal Reese Witherspoon a happy birthday with a naked mirror selfie.