diff --git a/src/proto/streams/streams.rs b/src/proto/streams/streams.rs
index f402f49f..33bcfd67 100644
--- a/src/proto/streams/streams.rs
+++ b/src/proto/streams/streams.rs
@@ -478,6 +478,11 @@ impl Inner {
 
         let stream = self.store.resolve(key);
 
+        if stream.is_pending_open {
+            proto_err!(conn: "recv_headers: received frame on idle stream {:?}", id);
+            return Err(Error::library_go_away(Reason::PROTOCOL_ERROR));
+        }
+
         if stream.state.is_local_error() {
             // Locally reset streams must ignore frames "for some time".
             // This is because the remote may have sent trailers before
@@ -638,6 +643,11 @@ impl Inner {
             }
         };
 
+        if stream.is_pending_open {
+            proto_err!(conn: "recv_reset: received frame on idle stream {:?}", id);
+            return Err(Error::library_go_away(Reason::PROTOCOL_ERROR));
+        }
+
         let mut send_buffer = send_buffer.inner.lock().unwrap();
         let send_buffer = &mut *send_buffer;
 
@@ -670,6 +680,11 @@ impl Inner {
             // The remote may send window updates for streams that the local now
             // considers closed. It's ok...
             if let Some(mut stream) = self.store.find_mut(&id) {
+                if stream.is_pending_open {
+                    proto_err!(conn: "recv_window_update: received frame on idle stream {:?}", id);
+                    return Err(Error::library_go_away(Reason::PROTOCOL_ERROR));
+                }
+
                 let res = self
                     .actions
                     .send
diff --git a/tests/h2-tests/tests/client_request.rs b/tests/h2-tests/tests/client_request.rs
index 7ad5ee85..66059969 100644
--- a/tests/h2-tests/tests/client_request.rs
+++ b/tests/h2-tests/tests/client_request.rs
@@ -2075,3 +2075,84 @@ impl MockH2 for mock_io::Builder {
             .read(SETTINGS_ACK)
     }
 }
+
+/// RFC 9113 S5.1: "Receiving any frame other than HEADERS or PRIORITY on a
+/// stream in [idle] state MUST be treated as a connection error of type
+/// PROTOCOL_ERROR."
+#[tokio::test]
+async fn frame_on_pending_open_stream_is_conn_error() {
+    h2_support::trace_init!();
+
+    for scenario in 0..5u8 {
+        let (io, mut srv) = mock::new();
+
+        let srv = async move {
+            let settings = srv
+                .assert_client_handshake_with_settings(frames::settings().max_concurrent_streams(1))
+                .await;
+            assert_default_settings!(settings);
+
+            // 3. Receive stream 1 HEADERS.
+            srv.recv_frame(
+                frames::headers(1)
+                    .request("POST", "https://example.com/")
+                    .eos(),
+            )
+            .await;
+
+            idle_ms(50).await;
+
+            // 4. Send a frame targeting stream 3, whose HEADERS haven't
+            //    been transmitted since it's pending. This is illegal.
+            match scenario {
+                0 => {
+                    srv.send_frame(frames::reset(3).reason(h2::Reason::NO_ERROR))
+                        .await
+                }
+                1 => {
+                    srv.send_frame(frames::reset(3).reason(h2::Reason::CANCEL))
+                        .await
+                }
+                2 => srv.send_frame(frames::window_update(3, 1024)).await,
+                3 => srv.send_frame(frames::headers(3).response(200).eos()).await,
+                4 => srv.send_frame(frames::data(3, &b"hello"[..])).await,
+                _ => unreachable!(),
+            }
+
+            // 5. Client responds with GOAWAY(PROTOCOL_ERROR).
+            srv.recv_frame(frames::go_away(0).protocol_error()).await;
+        };
+
+        let client = async move {
+            let (mut client, mut conn) = client::Builder::new()
+                .initial_max_send_streams(1)
+                .handshake::<_, Bytes>(io)
+                .await
+                .unwrap();
+
+            // 1. Stream 1 fills the concurrent slot
+            let request = Request::builder()
+                .method(Method::POST)
+                .uri("https://example.com/")
+                .body(())
+                .unwrap();
+            let (_resp1, _) = client.send_request(request, true).unwrap();
+            client = conn.drive(client.ready()).await.unwrap();
+
+            // 2. Stream 3 is queued
+            let request = Request::builder()
+                .method(Method::POST)
+                .uri("https://example.com/")
+                .body(())
+                .unwrap();
+            let (_resp3, _) = client.send_request(request, true).unwrap();
+
+            // 6. Connection error propagates to poll_ready.
+            conn.drive(client.ready())
+                .await
+                .expect_err("connection error");
+        };
+
+        join(srv, client).await;
+    }
+}