diff --git a/internal/witness/witness.go b/internal/witness/witness.go
index 8419eaf..dfbe80f 100644
--- a/internal/witness/witness.go
+++ b/internal/witness/witness.go
@@ -301,7 +301,10 @@ func (w *Witness) serveAddCheckpoint(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 	switch err {
-	case errUnknownLog, errInvalidSignature:
+	case errUnknownLog:
+		http.Error(rw, err.Error(), http.StatusNotFound)
+		return
+	case errInvalidSignature:
 		http.Error(rw, err.Error(), http.StatusForbidden)
 		return
 	case errBadRequest: