Error when trying to create a list property with object extending BlockData (CMS 12.29.1/Commerce 14.21.0)

Hello,

I'm trying to create a simple list property as described in this documentation guide: https://docs.developers.optimizely.com/content-management-system/docs/property-value-list

I have created the following code on an existing class in the project which is basically the same as the ContactBlock example in the guide:

...
[ContentType(AvailableInEditMode = false, GUID = "E7F0EB9A-3A55-408B-8FB7-344487C0C27B")]
public class TestBlockData : BlockData
{
    public virtual string TestField { get; set; }
}
[CatalogContentType( 
    GUID =  "D95D7056-4198-452B-9E4E-D315B9D141C6",
    DisplayName = "Dynamic Bundle Product",
    Description = "Provides options for customer to dynamically assemble its own bundle and get discounts for it.")]
public class DynamicBundleProduct2 : ProductContent, IProductRecommendations, IBrownellsContent, IBrownellsSeoProduct,IHideSubscribeToNewsletterContent
{
    //public override string ClassTypeId { get; }
    [Display(Name = "Bundle Items", GroupName = "Bundles")]
    public virtual IList<TestBlockData> TestListProperty { get; set; }
    ...
    //Rest of the class code
}

And when I start the project I get the following error:

I have tried the following:

• Changing GUID on both classes
• Changing main class name
• Upgrading all Epi packages to latest version

Please advise on possible solutions