error "markup in the document must be well-formed" after updating RDF/XML file using Jena

First, I was able to create OntModel model with model.read(owlinputstream,"RDF/XML") from an existing ontology that I have created using Protege with RDF/XML format, and even reading owl resources (e.g. model.getOntClass, ...) with no errors. But when I update the ontology using Jena methods (e.g. OntClass newparent= model.createClass(namespaceonto+"Parent"); newparent.createIndividual(namespaceonto+"Paul");) and try to re-read the file in model again: model.read(owlinputstream,"RDF/XML"), I receive the following error: Exception in thread "main" org.apache.jena.riot.RiotException: [line: 806, col: 2 ] The markup in the document following the root element must be well-formed...

Could anyone help me please in solving the issue. Thanks