diff --git a/src/syscall/zx/fidl/ctx_and_header.go b/src/syscall/zx/fidl/ctx_and_header.go
index 916d17b..78e9096 100644
--- a/src/syscall/zx/fidl/ctx_and_header.go
+++ b/src/syscall/zx/fidl/ctx_and_header.go
@@ -68,7 +68,7 @@
 func (ctx MarshalerContext) NewHeader() MessageHeader {
 	var flagByte0 uint8
 	if ctx.UseV2WireFormat {
-		flagByte0 = 1
+		flagByte0 = FidlV2WireFormatFlagMask
 	}
 	return MessageHeader{
 		Flags: [3]uint8{flagByte0, 0, 0},