diff --git a/addons/mail/static/src/js/m2x_avatar_user.js b/addons/mail/static/src/js/m2x_avatar_user.js
index 76ec83c440f..0d3d4fa4496 100644
--- a/addons/mail/static/src/js/m2x_avatar_user.js
+++ b/addons/mail/static/src/js/m2x_avatar_user.js
@@ -29,7 +29,7 @@ const M2XAvatarMixin = {
         if (!this.supportedModels.includes(this.field.relation)) {
             throw new Error(`This widget is only supported on many2one and many2many fields pointing to ${JSON.stringify(this.supportedModels)}`);
         }
-        this.className += ' o_clickable_m2x_avatar';
+        this.className = `${this.className || ''} o_clickable_m2x_avatar`.trim();
     },
 
     //--------------------------------------------------------------------------