diff --git a/tools/http2_interop/http2interop.go b/tools/http2_interop/http2interop.go
index 1276a71afc9..fa113961f2a 100644
--- a/tools/http2_interop/http2interop.go
+++ b/tools/http2_interop/http2interop.go
@@ -150,7 +150,8 @@ func testUnknownFrameType(ctx *HTTP2InteropCtx) error {
 	}
 
 	// Write a bunch of invalid frame types.
-	for ft := ContinuationFrameType + 1; ft != 0; ft++ {
+	// Frame number 11 is the upcoming ALTSVC frame, and should not be tested.
+	for ft := ContinuationFrameType + 2; ft != 0; ft++ {
 		fh := &UnknownFrame{
 			Header: FrameHeader{
 				Type: ft,