diff --git a/src/modules/Elsa.Workflows.Core/Serialization/Converters/PolymorphicObjectConverter.cs b/src/modules/Elsa.Workflows.Core/Serialization/Converters/PolymorphicObjectConverter.cs
index 838dae398..f4c280f97 100644
--- a/src/modules/Elsa.Workflows.Core/Serialization/Converters/PolymorphicObjectConverter.cs
+++ b/src/modules/Elsa.Workflows.Core/Serialization/Converters/PolymorphicObjectConverter.cs
@@ -242,6 +242,15 @@ public class PolymorphicObjectConverter : JsonConverter<object>
             return;
         }
 
+        // If the value was serialized as a primitive by another converter,
+        // write it directly instead of assuming an object structure.
+        if (jsonElement.ValueKind != JsonValueKind.Object &&
+            jsonElement.ValueKind != JsonValueKind.Array)
+        {
+            jsonElement.WriteTo(writer);
+            return;
+        }
+
         writer.WriteStartObject();
 
         if (jsonElement.ValueKind == JsonValueKind.Array)
diff --git a/test/integration/Elsa.Workflows.IntegrationTests/Serialization/Polymorphism/Tests.cs b/test/integration/Elsa.Workflows.IntegrationTests/Serialization/Polymorphism/Tests.cs
index fefa1734d..6278555f8 100644
--- a/test/integration/Elsa.Workflows.IntegrationTests/Serialization/Polymorphism/Tests.cs
+++ b/test/integration/Elsa.Workflows.IntegrationTests/Serialization/Polymorphism/Tests.cs
@@ -87,4 +87,30 @@ public class Tests
         options.Converters.Add(new PolymorphicObjectConverterFactory(new WellKnownTypeRegistry()));
         return options;
     }
+
+    [Fact(DisplayName = "Types with custom converters that serialize to primitives are serialized as primitives")]
+    
+    public void CustomConverterProducingPrimitive_IsSerializedAsPrimitive()
+    {
+        var model = new MyNumber { Number = 123UL };
+        var options = GetSerializerOptions();
+        var expectedJson = "123";
+        var json = JsonSerializer.Serialize<object>(model, options);
+        Assert.Equal(expectedJson, json);
+    }
+
+    [JsonConverter(typeof(MyNumberConverter))]
+    private struct MyNumber
+    {
+        public ulong Number { get; init; }
+    }
+
+    private class MyNumberConverter : JsonConverter<MyNumber>
+    {
+        public override MyNumber Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+            => new() { Number = reader.GetUInt64() };
+
+        public override void Write(Utf8JsonWriter writer, MyNumber value, JsonSerializerOptions options)
+            => writer.WriteNumberValue(value.Number);
+    }
 }
\ No newline at end of file