System.InvalidOperationException: 'Start and end inlines should belong to paragraph in one block container.'

        static void Main(string[] args)
        {
            DocxFormatProvider _DocXProvider = new DocxFormatProvider();
            string outputFile = $@"..\..\MergedResult.docx";
            File.Delete(outputFile); 

            RadFlowDocument target = _DocXProvider.Import(File.ReadAllBytes(@"..\..\HeaderFooter.docx"));
            RadFlowDocument source = _DocXProvider.Import(File.ReadAllBytes(@"..\..\SubHeaderFooter.docx"));
            RadFlowDocument contentSource = _DocXProvider.Import(File.ReadAllBytes(@"..\..\ContentOnly.docx"));

            MergeHeaders(target, source);
            MergeFooters(target, source);


            MergeContent(target, source);
            MergeContent(target, contentSource);
            System.IO.File.WriteAllBytes(outputFile, _DocXProvider.Export(target));
            Process.Start(outputFile);
        }

        private static void MergeContent(RadFlowDocument target, RadFlowDocument source)
        {
            DocumentElementImporter importer = new DocumentElementImporter(target, source, ConflictingStylesResolutionMode.UseTargetStyle);

            foreach (Telerik.Windows.Documents.Flow.Model.Section section in source.Sections)
            {
                foreach (BlockBase block in section.Blocks)
                {
                    BlockBase importBlock = importer.Import(block);
                    target.Sections.Last().Blocks.Add(importBlock);
                }
            }
        }

        private static void MergeHeaders(RadFlowDocument target, RadFlowDocument source)
        {
            Header targetHeader = target.Sections.First().Headers.Default;
            Header sourceHeader = source.Sections.First().Headers.Default;

            DocumentElementImporter importer = new DocumentElementImporter(target, source, ConflictingStylesResolutionMode.UseTargetStyle);
            foreach (BlockBase block in sourceHeader.Blocks)
            {
                BlockBase importedBlock = importer.Import(block);
                targetHeader.Blocks.Add(importedBlock);
            }
        }

        private static void MergeFooters(RadFlowDocument target, RadFlowDocument source)
        {
            Footer targetFooter = target.Sections.First().Footers.Default;
            Footer sourceFooter = source.Sections.First().Footers.Default;

            DocumentElementImporter importer = new DocumentElementImporter(target, source, ConflictingStylesResolutionMode.UseTargetStyle);
            int i = 0;
            foreach (BlockBase block in sourceFooter.Blocks)
            {
                BlockBase importedBlock = importer.Import(block);
                targetFooter.Blocks.Insert(i++, importedBlock);
            }
        }