error, when try to get a join cursor count

I have a primary db, and two secondary db.
do a join of two cursors of the two secondary db.
get the selected results from primary db correctly
but it failed when I try to get count of the join cursor.

thanks

		Db pdb(...);
		Db  sdb1(...);
		pdb.associate(NULL, &sdb1, getKey1, 0);
		Db  sdb2(...);
		pdb.associate(NULL, &sdb2, getKey2, 0);

		Dbt data;

		string t("aaa");
		Dbt key((void*) t.c_str(),(u_int32_t) t.length() +1);
		Dbc *cursor1;
		sdb1.cursor(NULL,&cursor1,0);
		int retl = cursor1->get(&key, &data, DB_SET);
		db_recno_t recno;
		cursor1->count(&recno,0);
		cout<<"#records found "<<recno<<endl;            // no problem here

		string t2("asdf");
		Dbt key((void*) t2.c_str(),(u_int32_t) t.length() +1);
		Dbc *cursor2;
		sdb2.cursor(NULL,&cursor1,0);
		int ret2 = cursor1->get(&key, &data, DB_SET);
		db_recno_t recno2;
		cursor2->count(&recno2,0);
		cout<<"#records found "<<recno2<<endl;   // no problem here


		// Set up the cursor array
		Dbc *carray[3];
		carray[0] = cursor1;
		carray[1] = cursor2;
		carray[2] = NULL;

		// Create the join
		Dbc *join_curs;
		int ret = pdb.join(carray, &join_curs, 0) ;
		if (!ret)
		{
			// Iterate using the join cursor
			while ((ret = join_curs->get(&key, &data, 0)) == 0) {
                                  //get selected record from pdb, no
problem
			}

		       db_recno_t recno;
			join_curs->count(&recno,0);      // error
here !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!??????????
			cout<<recno<<endl;

			if (cursor1 != NULL) cursor1->close();
			if (cursor2 != NULL) cursor2->close();
			if (join_curs != NULL) join_curs->close();
		}
		else
			cout<<0<<endl;

	}catch (DbException &e){
		cout<<e.what()<<endl;
		return (e.get_errno());
	}catch(exception &e){
		cout<<e.what()<<endl;
		return (-1);
	}
	 return true;