Word Exception 0x800A17EA -action invalid outside block-level XML

[Petrowi]

This code
wdDocument.Content.FormattedText = tempWdDoc.Content.FormattedText;

Throws an exception:

System.Runtime.InteropServices.COMException (0x800A17EA): This action is
not valid outside of a block-level XML element.
at Microsoft.Office.Interop.Word.Range.set_FormattedText(Range prop)


Both documents have XML markup in them and opened separately look ok.

Any ideas what does the error mean or what causes it?

Thank you.