false syntax error help please

Hi,
In the first email definition, the To: field is set up as this :
[hidden-username]<[hidden-useremail]>
Both fields are listed in the list of usable fields above the form, and the form works.
Yet I have this annoying message just below the To: field stating in red “Ce champ a des erreurs de syntaxe. Comment corriger cela ?”

Here is a screenshot : https://e-nable.fr/tmp/screenshot.png

Thanks for your help, I really want to get rid of this message.
Thierry