Kotlin mutators

I'm using Ektorp as CouchDB "ORM" but this question seems to be more general than that. Could someone explain what is the difference between

@JsonInclude(NON_NULL)
abstract class AbstractEntity(
        id: String?,
        revision: String?
) {

    @JsonProperty("_id")
    val id: String? = id

    @JsonProperty("_rev")
    val revision: String? = revision

}

and

@JsonInclude(NON_NULL)
abstract class AbstractEntity(
        @JsonProperty("_id")
        val id: String? = null,
        @JsonProperty("_rev")
        val revision: String? = null
)

?

For the first case Ektorp does not complain but for the second it says:

org.ektorp.InvalidDocumentException: Cannot resolve revision mutator in class com.scherule.calendaring.domain.Meeting

    at org.ektorp.util.Documents$MethodAccessor.assertMethodFound(Documents.java:165)
    at org.ektorp.util.Documents$MethodAccessor.<init>(Documents.java:144)
    at org.ektorp.util.Documents.getAccessor(Documents.java:113)
    at org.ektorp.util.Documents.getRevision(Documents.java:77)
    at org.ektorp.util.Documents.isNew(Documents.java:85)

In addition to that if I do

@JsonInclude(NON_NULL)
abstract class AbstractEntity(
        @JsonProperty("_id")
        var id: String? = null,
        @JsonProperty("_rev")
        var revision: String? = null
)

I get:

org.ektorp.DbAccessException: com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "_id" (class com.scherule.calendaring.domain.Meeting), not marked as ignorable

The question is in what way the first code snippet is different from the second? In the first case Ektorp "thinks" there is a mutator but not in the second...

Here's the code snippet which Ektorp seems to use when locating a method (setId, setRevision).

private Method findMethod(Class<?> clazz, String name,
                Class<?>... parameters) throws Exception {
            for (Method me : clazz.getDeclaredMethods()) {
                if (me.getName().equals(name)
                        && me.getParameterTypes().length == parameters.length) {
                    me.setAccessible(true);
                    return me;
                }
            }
            return clazz.getSuperclass() != null ? findMethod(
                    clazz.getSuperclass(), name, parameters) : null;
        }