我正在使用confluent 4.1.1，我尝试了几个聚合命令，但总是返回npe和group by：

ksql> select cust_id, count(*) as sum_sub from TBL_SUBSCRIBER GROUP BY cust_id;
 ServerError:java.lang.NullPointerException
ksql> 

ksql> describe extended TBL_SUBSCRIBER;

Type                 : TABLE
Key field            : SUB_ID
Timestamp field      : Not set - using <ROWTIME>
Key format           : STRING
Value format         : DELIMITED
Kafka output topic   : subscriber (partitions: 10, replication: 3)

 Field               | Type                      
 ROWTIME             | BIGINT           (system) 
 ROWKEY              | VARCHAR(STRING)  (system) 
 SUB_ID              | VARCHAR(STRING)           
 CONTRACT_ID         | VARCHAR(STRING)           
 CUST_ID             | VARCHAR(STRING)           
 ACCOUNT_ID          | VARCHAR(STRING)           
 TELECOM_SERVICE_ID  | VARCHAR(STRING)

请帮帮我！