Async await & embedded interop type error

Question

I got method which runs some word document report generation.

I tought that there is no way to do it synchronously, so, I decided to make async await method for that.

But for some resaon Visual Studio outputs this error

Cannot be used across assembly boundaries because it has a generic type parameters that is an embedded interop type

Embed Interop Types set to false and if I use simple synchronous method its seems okay

Code of functions is here

        public Document GenerateReport(AptTemplateType type)
        {
            try
            {
                var builder = _curentrBuilderFactory.GetBuilder(type);

                var generatedDocument = builder.CreateDocument();
                return generatedDocument;
            }
            catch (Exception e)
            {

                throw;
            }
        }

        public async Task<Document> GenerateReportAsync(AptTemplateType type)
        {
            WordDocumentBuilder builder;

            try
            {
                builder = _curentrBuilderFactory.GetBuilder(AptTemplateType.UseCaseModelSurvey);
                var generatedDocument = await Task.Run(() => builder.CreateDocument());
                return generatedDocument;
            }
            catch (Exception e)
            {

                throw;
            }
        }

UPD:

CreateDocument simple defenition:

public virtual Document CreateDocument()
{
    return WordApplication.Documents.Add(ref OMissing, ref OMissing, ref OMissing, ref OMissing);
}