diff --git a/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2FrameCodec.java b/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2FrameCodec.java
index d571ba496b..b4dacf0d70 100644
--- a/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2FrameCodec.java
+++ b/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2FrameCodec.java
@@ -548,6 +548,10 @@ public class Http2FrameCodec extends Http2ConnectionHandler {
         @Override
         public void onUnknownFrame(
                 ChannelHandlerContext ctx, byte frameType, int streamId, Http2Flags flags, ByteBuf payload) {
+            if (streamId == 0) {
+                // Ignore unknown frames on connection stream, for example: HTTP/2 GREASE testing
+                return;
+            }
             onHttp2Frame(ctx, new DefaultHttp2UnknownFrame(frameType, flags, payload)
                     .stream(requireStream(streamId)).retain());
         }
diff --git a/codec-http2/src/test/java/io/netty/handler/codec/http2/Http2FrameCodecTest.java b/codec-http2/src/test/java/io/netty/handler/codec/http2/Http2FrameCodecTest.java
index 1f382584dd..b40486babc 100644
--- a/codec-http2/src/test/java/io/netty/handler/codec/http2/Http2FrameCodecTest.java
+++ b/codec-http2/src/test/java/io/netty/handler/codec/http2/Http2FrameCodecTest.java
@@ -375,6 +375,18 @@ public class Http2FrameCodecTest {
         assertEquals(0, frame.refCnt());
     }
 
+    @Test
+    public void unknownFrameTypeOnConnectionStream() throws Exception {
+        // handle the case where unknown frames are sent before a stream is created,
+        // for example: HTTP/2 GREASE testing
+        ByteBuf debugData = bb("debug");
+        frameInboundWriter.writeInboundFrame((byte) 0xb, 0, new Http2Flags(), debugData);
+        channel.flush();
+
+        assertEquals(0, debugData.refCnt());
+        assertTrue(channel.isActive());
+    }
+
     @Test
     public void goAwayLastStreamIdOverflowed() throws Exception {
         frameInboundWriter.writeInboundHeaders(5, request, 31, false);