Democratic presidential frontrunner Hillary Clinton will enjoy the support of thirteen female Democratic U.S. senators at her ‘Women For Hillary’ endorsement event Monday night in Washington, D.C.

But thirteen just might be her unlikely number. If her campaign aides are counting, there are actually fourteen female Democratic U.S. senators. And one of them is not showing up.

Massachusetts Sen. Elizabeth Warren, who has yet to endorse Clinton for president, is notably absent from the list of female senatorial guests who will attend the swanky event at Capitol Hill’s Hyatt Regency hotel.

Warren, who was pressured to run for the Democratic nomination but opted out of the race, paving the way for Clinton’s left-wing challenger Bernie Sanders, might be leaning toward a Sanders endorsement over Clinton.

“I’ve had many, many meetings with Elizabeth Warren,” Sanders recently said, and Sanders has been able to siphon off some support from onetime Warren-for-president supporters.

Female Senate Democrats, minus Warren, recently put out an ad for the Clinton campaign in which Sen. Claire McCaskill insulted housewives, snarking that a Republican president would relegate women to “June Cleaver” (read: traditional homemaker) style roles in America.

Neither Warren’s office nor the Clinton campaign immediately returned a request for comment for this report.