The People's Association made a mistake and used an old invitation list instead of an updated one.
That is how it ended up inviting and then uninviting former People's Action Party Member of Parliament and presidential candidate Dr Tan Cheng Bock to a Chinese New Year garden party.
The party is an annual event for past and present grassroots leaders and this year's is taking place this Sunday evening at the Istana.
That was the explanation given by Mr Lim Swee Say, the deputy chairman of the PA and labour chief, in response to media queries on Friday hours after Dr Tan posted on his Facebook page a status update on being uninvited.
"It is most unfortunate that PA made the mistake of using the old list instead of the updated list. We are sorry for the error, and this is why I phoned Mr Tan and the other affected invitees personally to explain the mistake, and followed up with personal e-mails to apologise again," Mr Lim said.
He added that the list of invitees is reviewed and refreshed periodically to allow a wider base of people who serve the community to attend the yearly reception.
"The most recent review was conducted a few months ago. Instead of inviting all ex-advisers to grassroots organisations repeatedly for 20, 30 years or even longer, we decided to limit the list to only ex-advisers who stood down in the immediate past general election," he added.
Mr Lim did not say how many invitees were affected by this mistake, or who they are.
He added that he was "heartened that Mr Tan very graciously accepted my explanation over the phone" and is "surprised that he now brings this up publicly as an issue".
Dr Tan's post has gathered almost 2,000 likes and more than 800 shares since it was put online on Friday morning.
He told The Straits Times that he decided to write about in on Facebook because many grassroots leaders had asked if he would be attending the event.