diff --git a/backend/src/Squidex.Infrastructure/UniqueConstraintException.cs b/backend/src/Squidex.Infrastructure/UniqueConstraintException.cs
index 96570ad79..98825ca2f 100644
--- a/backend/src/Squidex.Infrastructure/UniqueConstraintException.cs
+++ b/backend/src/Squidex.Infrastructure/UniqueConstraintException.cs
@@ -6,6 +6,7 @@
 // ==========================================================================
 
 using System;
+using System.Runtime.Serialization;
 
 namespace Squidex.Infrastructure
 {