Why can't I save an instance of this enterprise object?

i'm using some very basic code to create an save an enterprise object.
i've tried it with another object and it works fine. i've never see
the error below. does anyone know what causes it?

many thanks,
matt

protected abHistorySet testSet;
		
    public exhibitionsOpening(WOContext context) {
        super(context);

		testSet = new abHistorySet();
		session().defaultEditingContext().insertObject(testSet);
		session().defaultEditingContext().saveChanges();

}

Error:  	 java.lang.IllegalStateException: Unable to find entity for
object {values = {status = ; values = (); entityName = ; rowid = ;
dateCreated = ; entityID = ; }; this = ">"; }

Reason: 	Unable to find entity for object {values = {status =
<com.webobjects.foundation.NSKeyValueCoding$Null>; values = ();
entityName = <com.webobjects.foundation.NSKeyValueCoding$Null>; rowid
= <com.webobjects.foundation.NSKeyValueCoding$Null>; dateCreated =
<com.webobjects.foundation.NSKeyValueCoding$Null>; entityID =
<com.webobjects.foundation.NSKeyValueCoding$Null>; }; this =
"<abHistorySet abc94 <EOTemporaryGlobalID: 0 0 -64 -88 0 40 0 0 -61 85
1 0 0 0 1 9 10 53 -14 -92 53 -42 -78 67>>"; }