The Girls creator and star was sure the fun. rocker was planning to pop the question, only to discover he had actually been complaining about her to his fashion designer sibling, Rachel Antonoff.
"I thought Jack was furtively planning a proposal to me with his sister," she told fans on Twitter. "And he just admitted I actually caught him 'talking s**t' about me!"
The funny turn of events made Lena burst into laughter - preventing both her and Jack from getting to sleep.
"Now I literally can't sleep cuz (sic) I'm laughing too hard and I am also keeping him awake," she added. "5.5 years, no rock and you know what? It's real nice."
She went on to try and explain how she managed to confuse the chatter: "Also they were huddled and I heard him say 'Lena' and 'finger' so obviously it seemed like an engagement ring talk," she shared.
The 31-year-old then made it clear to Jack what kind of jewel she was expecting if and when he eventually does propose.
"I want Cardi B's ring," she stated, referencing the eight-carat stunner Migos rapper Offset presented to the Bodak Yellow MC when he asked her to marry him on stage last week.
Lena has previously insisted she doesn't need to be married to feel secure in her relationship with Jack, who she began dating in 2011, although she has started to plan out their nuptials in her head.
"Jack and I have talked about it (marriage)," she admitted to PrideSource last year, "and we've always said that when we get married we want our wedding party to just be our two sisters in tuxedos.
"Jack has a straight sister, I have a queer sister; they'd be our best men/women and we'll call it a day. That's our dream."