IllegalArgumentException in ConvertingPropertyEditorAdapter due to invalid BeanWrapper expression

Given this code:

public class Survey {
private Map<Question, Response> responses = new HashMap<Question,Response>();
...
}

public class QuestionConverter implements Converter<Integer, Question> {
...
}

<form:input path="responses[1].value" />

During rendering, the expression "responses[1].value" is processed correctly by Spring EL, which converts the integer 1 to a Question, then does a look up in the responses map, and then converts the "value" field to a String. At this point this should be all that is needed. However, the following exception occurs:

java.lang.IllegalArgumentException: The field type is required
at org.springframework.util.Assert.notNull(Assert.java:112)
at org.springframework.webflow.mvc.view.ConvertingPropertyEditorAdapter.<init>(ConvertingPropertyEditorAdapter.java:48)
at org.springframework.webflow.mvc.view.BindingModel.findSpringConvertingPropertyEditor(BindingModel.java:273)
at org.springframework.webflow.mvc.view.BindingModel.findEditor(BindingModel.java:183)
at org.springframework.web.servlet.support.BindStatus.<init>(BindStatus.java:125)
at org.springframework.web.servlet.tags.form.AbstractDataBoundFormElementTag.getBindStatus(AbstractDataBoundFormElementTag.java:174)

Spring MVC's BindStatus tries to find a PropertyEditor but the above expression cannot be processed by the BeanWrapperImpl because it fails to recognize that 1 needs to be converted to a Question.

It would be safe if we didn't create a ProperytEditor when the BeanWrapperImpl fails as Spring EL has already formatted the value correctly.