diff --git a/nns/nns_contract.go b/nns/nns_contract.go
index 9df2d18..816f56c 100644
--- a/nns/nns_contract.go
+++ b/nns/nns_contract.go
@@ -239,6 +239,9 @@ func IsAvailable(name string) bool {
 	ctx := storage.GetReadOnlyContext()
 	l := len(fragments)
 	if storage.Get(ctx, append([]byte{prefixRoot}, []byte(fragments[l-1])...)) == nil {
+		if l != 1 {
+			panic("panic should not happen")
+		}
 		return true
 	}
 
diff --git a/tests/nns_test.go b/tests/nns_test.go
index f19a448..51450bb 100644
--- a/tests/nns_test.go
+++ b/tests/nns_test.go
@@ -588,6 +588,7 @@ func TestNNSIsAvailable(t *testing.T) {
 	c := newNNSInvoker(t, false)
 
 	c.Invoke(t, true, "isAvailable", "com")
+	c.InvokeFail(t, "panic should not happen", "isAvailable", "aa.domain.zk")
 
 	refresh, retry, expire, ttl := int64(101), int64(102), int64(103), int64(104)
 	c.Invoke(t, true, "register",