If the text area for the contents is left empty, PRPage>>contents: receives a nil object and fails to parse it as a document. The reason is that the parser can only interpret string objects and not nil objects. PRDocumentParser parse: nil. The solution could be to assign an empty PRDocument if the value of aString is an nil object. PRPage>>contents: aString aString ifNil: [ self document: PRDocument new ] ifNotNil: [ self document: (self parserClass parse: aString) ].