can not delete one family

[Sindy]

Hi,
I want to delete one family in the java programe,
Here is my code:
-----------------------------

  Configuration conf=HBaseConfiguration.create();
  HBaseAdmin admin = new HBaseAdmin(conf);

  if (admin.tableExists(tableName)) {

  HTable table = new HTable(conf,tableName);
  Delete d=new Delete();
  admin.disableTable(tableName);
  d.deleteFamily(Bytes.toBytes(family));
  table.delete(d);
  admin.enableTable(tableName);
  System.out.println("delete family ok!");
  }
  System.out.println("no this table ok!");
  }

-----------------------------
But it shows the exception:

Exception in thread "main" java.lang.IllegalArgumentException: Row is null
 at org.apache.hadoop.hbase.KeyValue.createByteArray(KeyValue.java:434)
 at org.apache.hadoop.hbase.KeyValue.<init>(KeyValue.java:399)
 at org.apache.hadoop.hbase.KeyValue.<init>(KeyValue.java:368)
 at org.apache.hadoop.hbase.KeyValue.<init>(KeyValue.java:347)
 at org.apache.hadoop.hbase.KeyValue.<init>(KeyValue.java:317)
 at org.apache.hadoop.hbase.client.Delete.deleteFamily(Delete.java:169)
 at org.apache.hadoop.hbase.client.Delete.deleteFamily(Delete.java:148)
 at DBOper.deleteOneFamily(DBOper.java:618)
 at MainClass.main(MainClass.java:41)


Could you give me any advices? Thanks

-- 
Sindy