diff --git a/test_suite/tests/test_annotations.rs b/test_suite/tests/test_annotations.rs
index b3bd7a79..87eda501 100644
--- a/test_suite/tests/test_annotations.rs
+++ b/test_suite/tests/test_annotations.rs
@@ -2269,3 +2269,45 @@ fn test_internally_tagged_unit_enum_with_unknown_fields() {
         ],
     );
 }
+
+#[test]
+fn test_flattened_internally_tagged_unit_enum_with_unknown_fields() {
+    #[derive(Deserialize, PartialEq, Debug)]
+    struct S {
+        #[serde(flatten)]
+        x: X,
+        #[serde(flatten)]
+        y: Y,
+    }
+
+    #[derive(Deserialize, PartialEq, Debug)]
+    #[serde(tag = "typeX")]
+    enum X {
+        A,
+    }
+
+    #[derive(Deserialize, PartialEq, Debug)]
+    #[serde(tag = "typeY")]
+    enum Y {
+        B { c: u32 },
+    }
+
+    let s = S {
+        x: X::A,
+        y: Y::B { c: 0 },
+    };
+
+    assert_de_tokens(
+        &s,
+        &[
+            Token::Map { len: None },
+            Token::Str("typeX"),
+            Token::Str("A"),
+            Token::Str("typeY"),
+            Token::Str("B"),
+            Token::Str("c"),
+            Token::I32(0),
+            Token::MapEnd,
+        ],
+    );
+}