diff --git a/cmd-merge-cars.go b/cmd-merge-cars.go
index 9abbf181..6cf3b2fc 100644
--- a/cmd-merge-cars.go
+++ b/cmd-merge-cars.go
@@ -10,7 +10,26 @@ import (
 	"github.com/urfave/cli/v2"
 )
 
-const varintSize = 10
+const (
+	varintSize       = 10
+	nulRootCarHeader = "\x19" + // 25 bytes of CBOR (encoded as varint :cryingbear: )
+		// map with 2 keys
+		"\xA2" +
+		// text-key with length 5
+		"\x65" + "roots" +
+		// 1 element array
+		"\x81" +
+		// tag 42
+		"\xD8\x2A" +
+		// bytes with length 5
+		"\x45" +
+		// nul-identity-cid prefixed with \x00 as required in DAG-CBOR: https://github.com/ipld/specs/blob/master/block-layer/codecs/dag-cbor.md#links
+		"\x00\x01\x55\x00\x00" +
+		// text-key with length 7
+		"\x67" + "version" +
+		// 1, we call this v0 due to the nul-identity CID being an open question: https://github.com/ipld/go-car/issues/26#issuecomment-604299576
+		"\x01"
+)
 
 func newCmd_MergeCars() *cli.Command {
 	var outputFile string