Corrected the fix.

backend/src/Squidex.Domain.Apps.Entities.MongoDb/Assets/Visitors/FindExtensions.cs

@@ -46,7 +46,7 @@ namespace Squidex.Domain.Apps.Entities.MongoDb.Assets.Visitors
 
             if (parentId.HasValue)
             {
-                if (parentId.Value == DomainId.Empty)
+                if (parentId == DomainId.Empty)
                 {
                     filters.Add(
                         Filter.Or(

backend/src/Squidex.Infrastructure/DomainId.cs

@@ -31,7 +31,7 @@ namespace Squidex.Infrastructure
                 return null;
             }
 
-            return new DomainId(value);
+            return Create(value);
         }
 
         public static DomainId Create(string value)

backend/tests/Squidex.Infrastructure.Tests/DomainIdTests.cs

@@ -91,6 +91,30 @@ namespace Squidex.Infrastructure
             Assert.Equal(text, domainId.ToString());
         }
 
+        [Fact]
+        public void Should_compare_with_nullable()
+        {
+            DomainId? value = DomainId.Empty;
+
+            Assert.True(value == DomainId.Empty);
+        }
+
+        [Fact]
+        public void Should_compare_with_nullable2()
+        {
+            DomainId? value = DomainId.Create(Guid.Empty.ToString());
+
+            Assert.True(value == DomainId.Empty);
+        }
+
+        [Fact]
+        public void Should_compare_with_non_shared_nullable()
+        {
+            DomainId? value = DomainId.Create("0");
+
+            Assert.True(value == DomainId.Create("0"));
+        }
+
         [Fact]
         public void Should_make_correct_equal_comparisons()
         {