"I'm gonna lose my job over this brilliant book @lenadunham @jlo #jlotookmybook #backtowork #notthatkindofgirl," Anderson captioned a video working out alongside Lopez.
In the Instagram clip, the "Booty" singer, 45, and Anderson, 39, start doing arm lifts with free weights until the trainer gets distracted. Squatting down she picked up the Girls' creator's new memoir.
When Lopez (and her incredible six pack) got annoyed by her fitness guru's absence, Anderson quickly replied, "I'm so sorry. It's just so good, I can't put it down! It's Lena's new book."
The whole scene seemed pretty staged, but that didn't prevent Dunham from thanking her famous fans for their support.
"Oh hey @TracyAnderson @JLo you are the pep squad of my dreams <3," she tweeted on Oct. 11.
Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics and more delivered straight to your inbox!