diff --git a/mtc.go b/mtc.go
index 767b01a..6b4270f 100644
--- a/mtc.go
+++ b/mtc.go
@@ -798,7 +798,7 @@ func (p *CAParams) NewTreeHeads(prevHeads, root []byte) ([]byte, error) {
 			len(root),
 		)
 	}
-	return append(root, prevHeads[HashLen:]...), nil
+	return append(root, prevHeads[:expected-HashLen]...), nil
 }
 
 func (batch *Batch) Anchor() TrustAnchorIdentifier {