We are tring to save document with ContentType properties in SharePoint 2013 Document Library programatically. so we are using SaveAs(<path to sharepoint document library>) to save the document so that document will be linked to SharePoint document library even after saving the document first time. but the problem here is it doesn't allow to save Lookup field value, its works fine for multilookup fields but not for single lookup. when we try to set correct value to lookup property then it just crash the document or in other words document gets closed automatically.

Any help would be greatly appreciated.