Conversion error for select items of type String

An error occurs when using select items of type String in combination with the OmniFaces SelectItemsIndexConverter. MyFaces seems to attempt to convert the submitted value twice, which leads to a NumberFormatException on the second try.

I suspect the following if-statement in org.apache.myfaces.core.api.shared.SelectItemsUtil to be the culprit, but of course have no idea why it is needed in other cases or how to fix the problem:

Please see the attached reproducer, which produces the error when selecting a value in the SelectOneMenu when run with 

mvn clean jetty:run -Pmyfaces40