Google Appengine for Java: tracsactionは1つのエンティティグループ内のみ。

Problem accessing /blobstore.jsp. Reason:

can't operate on multiple entity groups in a single transaction. found both Element {
type: "txnHead"
id: 0x130a7388166
}
and Element {
type: "txnBody"
id: 0x130a7388166
}

Caused by:

java.lang.IllegalArgumentException: can't operate on multiple entity groups in a single transaction. found both Element {
type: "txnHead"
id: 0x130a7388166
}
and Element {
type: "txnBody"
id: 0x130a7388166
}