the songs featured in the episode were initially composed by Sugar, who released her demo version of "I'm Just Your Problem" on her official Tumblr page. Her father, Rob, later released the demos for both "I'm Just Your Problem" and "My Best Friends in the World" on YouTube. "I'm Just Your Problem" was actually the last song written for the episode; Sugar was down to the last minute, and as such, she argued that her emotionally exhausted state informed her songwriting. The song was inspired by an incident that Sugar had experienced with a former roommate. Although the two often fought, Sugar desperately wanted them to be friends despite not particularly liking her. Sugar transferred this situation onto Marceline, writing the song so that Marceline—while having issues with Princess Bubblegum—wanted to become her friend. "My Best Friends in the World" was written by Sugar as a celebration of the friendship she shared with her coworkers, especially her storyboarding partner, Muto. Ward later admitted on the commentary track for "What Was Missing" that, while he was editing the animatic for the episode, he started to cry because the songs featured in the episode were so full of emotion. Adventure Time composer, Tim Kiefer, played the instruments featured in the episode himself. To complement the tones of the bass and viola, he created percussion sounds with Game Boy loops, and then used an autochord for "Princess Bubblegum's melodies" to make "pretty, melodic swoops to accompany BMO's rigid, robotic patterns, loops, and structures." Reception "What Was Missing" first aired on Cartoon Network on September 26, 2011. The episode was viewed by 2.185 million viewers and scored a 0.4 Nielsen rating in the 18–49-year-old demographic. This means it was seen by 0.4 percent of all households aged 18 to 49 years old were watching television at the time of the episode's airing. Tyler Foster of DVD Talk called the episode a "highlight" of the Fionna & Cake DVD. Specifically, he praised the song "I'm Just Your Problem", calling it both a "fan favorite" and "significant as the moment I decided I was a fan". Dana Piccoli of AfterEllen.com enjoyed the way the episode "alluded to perhaps, more than platonic feelings between" Princess Bubblegum and Marceline, as well as the fact that "What Was Missing" was underlined by potential lesbian "subtext". The A.V. Club named the episode one of the ten additional installments of the