diff --git a/clap_derive/src/derives/args.rs b/clap_derive/src/derives/args.rs
index d6fecc01..04928db2 100644
--- a/clap_derive/src/derives/args.rs
+++ b/clap_derive/src/derives/args.rs
@@ -230,7 +230,7 @@ pub(crate) fn gen_augment(
                 let next_display_order = item.next_display_order();
                 let flatten_group_assert = if matches!(**ty, Ty::Option) {
                     quote_spanned! { kind.span()=>
-                        <#inner_type as clap::Args>::group_id().expect("cannot `#[flatten]` an `Option<Args>` with `#[group(skip)]");
+                        <#inner_type as clap::Args>::group_id().expect("cannot `#[flatten]` an `Option<Args>` with `#[group(skip)]`");
                     }
                 } else {
                     quote! {}
diff --git a/tests/derive/flatten.rs b/tests/derive/flatten.rs
index 95ad43e2..6a1414e6 100644
--- a/tests/derive/flatten.rs
+++ b/tests/derive/flatten.rs
@@ -257,7 +257,7 @@ fn docstrings_ordering_with_multiple_clap_partial() {
 }
 
 #[test]
-#[should_panic = "cannot `#[flatten]` an `Option<Args>` with `#[group(skip)]"]
+#[should_panic = "cannot `#[flatten]` an `Option<Args>` with `#[group(skip)]`"]
 fn flatten_skipped_group() {
     #[derive(clap::Parser, Debug)]
     struct Cli {