Add Nullable to list of allowed facets for edm:ReturnType element. Nullable="false" means that the function or action will never return a null value but instead fail with 4xx if it cannot compute a result. For functions that return a single entity 404 Not Found is the appropriate answer.

https://tools.oasis-open.org/version-control/browse/wsvn/odata/trunk/spec/schemas/edm.xsd?rev=368 https://tools.oasis-open.org/version-control/browse/wsvn/odata/trunk/spec/schemas/edm.xsd?rev=369 https://www.oasis-open.org/committees/download.php/49615/odata-v4.0-wd02-part3-csdl-2013-06-19.docx https://tools.oasis-open.org/version-control/browse/wsvn/odata/trunk/spec/schemas/MetadataService.edmx?rev=374