diff --git a/serde_derive/src/de.rs b/serde_derive/src/de.rs
index 7be6c4b9..7fa1030b 100644
--- a/serde_derive/src/de.rs
+++ b/serde_derive/src/de.rs
@@ -2476,7 +2476,9 @@ fn deserialize_map(
 
     let collected_deny_unknown_fields = if cattrs.has_flatten() && cattrs.deny_unknown_fields() {
         Some(quote! {
-            if let _serde::export::Some(_serde::export::Some((__key, _))) = __collect.into_iter().filter(Option::is_some).next() {
+            if let _serde::export::Some(_serde::export::Some((__key, _))) =
+                __collect.into_iter().filter(_serde::export::Option::is_some).next()
+            {
                 if let _serde::export::Some(__key) = __key.as_str() {
                     return _serde::export::Err(
                         _serde::de::Error::custom(format_args!("unknown field `{}`", &__key)));
diff --git a/test_suite/tests/test_gen.rs b/test_suite/tests/test_gen.rs
index 788e91b8..92bb0ae1 100644
--- a/test_suite/tests/test_gen.rs
+++ b/test_suite/tests/test_gen.rs
@@ -522,6 +522,13 @@ fn test_gen() {
     }
     assert::<FlattenWith>();
 
+    #[derive(Serialize, Deserialize)]
+    #[serde(deny_unknown_fields)]
+    struct FlattenDenyUnknown<T> {
+        #[serde(flatten)]
+        t: T,
+    }
+
     #[derive(Serialize, Deserialize)]
     struct StaticStrStruct<'a> {
         a: &'a str,