In the section on expressions will note that expressions on calculated, initial and answerExpression are expected to resolve to the same type as the question type, allowing for the [implicit conversions] ( http://hl7.org/fhirpath/#conversion ) defined in FHIRPath. The only exception is that an expression that resolves to a Resource can satisfy a question of type 'Reference', in which case the result will be treated as a Reference to the returned resource. Will also add a section called "Dealing with expression errors" In some cases, a Questionnaire may contain expressions that result in an error. This could be the result of an invalid expression, unexpected data, or an expression handling engine that does not handle some aspect of the expression provided. In such cases, Form Fillers SHOULD attempt to allow the user to fill out the QuestionnaireResponse in spite of the issue, though they SHOULD also warn the user that there was a problem with the Questionnaire. If expressions are involved in display (e.g. enableWhen) or validation logic, Form Fillers SHOULD prohibit the QuestionnaireResponse status from being set to 'completed' and provide an explanation of the problem to the user.