give a heads up

Kazuo Ishiguroの "Klara and the Sun" を読んでいます。
Her silhouette appeared in the doorway, making the kitchen even darker, and she said:
'Klara, I wanted to give you a heads up. After all, this involves you.'
'Yes?'
'Next Thursday, I've taken time off work. I'm going to drive Josie into town and we'll be staying overnight. We were just talking about it. Josie has an appointment.'

"give you a heads up" の意味を調べます。慣用句かなと思いましたが、辞書には "heads up" もしくは "heads-up" に次の説明がありました。

・Oxford English Dictionary: An advance warning of something.: That's been fixed - and thanks for the heads-up, Steven.
・Collins Dictionary: if you give someone a heads-up about something that is going to happen, you tell them about it before it happens.: When the airlines decided to enforce these rules without giving anyone a heads-up, it created a lot of problems for travelers.
・Cambridge English Dictionary: a warning that something is going to happen, usually so that you can prepare for it: This note is just to give you a heads-up that Vicky will be arriving next week.

本は既に図書館に返したので確認できませんが、辞書は "a heads-up" となっており、 "a heads up" だと複数形の "heads" に "a" が付いているようで違和感があります。私の転記ミスの可能性が大です。