Marc21Encoder fails with IllegalStateException on receiving leader entity

[cboehme]

The Marc21Encoder fails to process the a stream of

start-record 
start-entity "leader"
end-entity
end-record

It throws the following exception:

java.lang.IllegalStateException
	at org.metafacture.biblio.iso2709.RecordBuilder.requireInDataField(RecordBuilder.java:273)
	at org.metafacture.biblio.iso2709.RecordBuilder.endDataField(RecordBuilder.java:220)
	at org.metafacture.biblio.marc21.Marc21Encoder.endEntity(Marc21Encoder.java:159)