Britney Spears has blown off Madonna‘s invitation to join her onstage at one of the Material Girl’s recent Los Angeles shows.

“Considering Britney’s ongoing challenges, Madonna understands why she may have chosen not to accept,” a source exclusively tells Life & Style.

Last summer, it was reported that the “Vogue” singer, 65, wanted Spears, 42, to appear at one of her Los Angeles dates in March on the Celebration Tour, which is currently playing around the United States before heading to Mexico City. Madonna reportedly wanted to mark the 20th anniversary of their song, “Me Against the Music,” which was released in October 2003.

“It’s hard to be mad with Britney after all she’s been through,” the source adds of the “Toxic” singer, who separated from husband Sam Asghari last August.

The insider explains, “Madonna recognized the potential for significant press coverage and she would have loved to kiss Britney again onstage, like at the VMAs in 2003 — but it’s not the end of the world if it doesn’t happen!”

Though the Queen of Pop may not see Spears join her onstage during the remainder of her concert dates, the Celebration Tour still welcomed some very special guests since it kicked off last October in London.

Madonna’s youngest daughter Estere, 11, stole the show when she vogued to her mother’s chart-topping hits on opening night. As Estere lit up the stage, the Grammy winner and her oldest daughter, Lourdes Leon, 27, held up score sheets that read “10.”