diff --git a/httpcore5-h2/src/main/java/org/apache/hc/core5/http2/impl/nio/AbstractH2StreamMultiplexer.java b/httpcore5-h2/src/main/java/org/apache/hc/core5/http2/impl/nio/AbstractH2StreamMultiplexer.java
index 9bc5b6b15..11c5c39f7 100644
--- a/httpcore5-h2/src/main/java/org/apache/hc/core5/http2/impl/nio/AbstractH2StreamMultiplexer.java
+++ b/httpcore5-h2/src/main/java/org/apache/hc/core5/http2/impl/nio/AbstractH2StreamMultiplexer.java
@@ -915,12 +915,14 @@ private void consumeFrame(final RawFrame frame) throws HttpException, IOExceptio
                 if (streamId == 0) {
                     throw new H2ConnectionException(H2Error.PROTOCOL_ERROR, "Illegal stream id: " + streamId);
                 }
+
+                final ByteBuffer payload = frame.getPayload();
+                if (payload == null || payload.remaining() != 4) {
+                    throw new H2ConnectionException(H2Error.FRAME_SIZE_ERROR, "Invalid RST_STREAM frame payload");
+                }
+
                 final H2Stream stream = streams.lookupSeen(streamId);
                 if (stream != null) {
-                    final ByteBuffer payload = frame.getPayload();
-                    if (payload == null || payload.remaining() != 4) {
-                        throw new H2ConnectionException(H2Error.FRAME_SIZE_ERROR, "Invalid RST_STREAM frame payload");
-                    }
                     final int errorCode = payload.getInt();
                     if (errorCode == H2Error.NO_ERROR.getCode() && allowGracefulAbort(stream)) {
                         stream.abortGracefully();
diff --git a/httpcore5-h2/src/test/java/org/apache/hc/core5/http2/impl/nio/TestAbstractH2StreamMultiplexer.java b/httpcore5-h2/src/test/java/org/apache/hc/core5/http2/impl/nio/TestAbstractH2StreamMultiplexer.java
index ea16173a3..e792f0405 100644
--- a/httpcore5-h2/src/test/java/org/apache/hc/core5/http2/impl/nio/TestAbstractH2StreamMultiplexer.java
+++ b/httpcore5-h2/src/test/java/org/apache/hc/core5/http2/impl/nio/TestAbstractH2StreamMultiplexer.java
@@ -1931,4 +1931,39 @@ void testFirstPeerSettingsAckRejected() throws Exception {
         Assertions.assertEquals(H2Error.PROTOCOL_ERROR, H2Error.getByCode(ex.getCode()));
     }
 
+    @Test
+    void testInputRstStreamWithInvalidLengthOnUnseenStreamRejected() throws Exception {
+        Mockito.when(protocolIOSession.write(ArgumentMatchers.any(ByteBuffer.class)))
+                .thenAnswer(invocation -> {
+                    final ByteBuffer buffer = invocation.getArgument(0, ByteBuffer.class);
+                    final int remaining = buffer.remaining();
+                    buffer.position(buffer.limit());
+                    return remaining;
+                });
+        Mockito.doNothing().when(protocolIOSession).setEvent(ArgumentMatchers.anyInt());
+        Mockito.doNothing().when(protocolIOSession).clearEvent(ArgumentMatchers.anyInt());
+
+        final AbstractH2StreamMultiplexer mux = new H2StreamMultiplexerImpl(
+                protocolIOSession,
+                FRAME_FACTORY,
+                StreamIdGenerator.ODD,
+                httpProcessor,
+                CharCodingConfig.DEFAULT,
+                H2Config.custom().build(),
+                h2StreamListener,
+                () -> streamHandler);
+
+        mux.onConnect();
+        completeSettingsHandshake(mux);
+
+        final RawFrame badRst = new RawFrame(FrameType.RST_STREAM.getValue(), 0, 1, null);
+
+        final H2ConnectionException ex = Assertions.assertThrows(
+                H2ConnectionException.class,
+                () -> mux.onInput(ByteBuffer.wrap(encodeFrame(badRst))));
+
+        Assertions.assertEquals(H2Error.FRAME_SIZE_ERROR, H2Error.getByCode(ex.getCode()));
+    }
+
+
 }
\ No newline at end of file