Redis AWS ElastiCache无法增加NumberOfAvailabilityZones

Question

我正在尝试更改CloudFormation模板，但失败并出现错误：

INFO: stackname found was elas-redis
INFO: creating change set elas-redis-2018-06-14-125050
INFO: waited for change set for elas-redis, status CREATE_COMPLETE -prd-eu-west-1/elas-redis/redis.json
INFO: Modify - RedisDNSRecord (Properties.RecordSets)
INFO: Modify - RedisReplicationGroup (Properties.AutomaticFailoverEnabled,Properties.NumCacheClusters,Properties.NumCacheClusters,Tags,Properties.NumCacheClusters)
INFO: Modify - RedisSubnetGroup (Properties.SubnetIds)
INFO: waited 30s for update of elas-redis to complete, status UPDATE_ROLLBACK_COMPLETE

Reacting with eyes
INFO: destructive rolling update stack elas-redis release redis0.3.2 with platform release 0.6.3
INFO: changing parameter HighAvailability from 'no' to 'yes'
INFO: changing parameter NumberOfAvailabilityZones from '2' to '3'
INFO: about to perform a rolling update on the instances .
INFO: no new resources to add to the stack.
INFO: update stack elas-redis release redis0.3.2 with platform release 0.6.3
INFO: changing parameter HighAvailability from 'no' to 'yes'
INFO: changing parameter NumberOfAvailabilityZones from '2' to '3'

In cloudformation:

The following resource(s) failed to update: [RedisReplicationGroup].
12:51:04 UTC+0200 UPDATE_FAILED AWS::ElastiCache::ReplicationGroup RedisReplicationGroup Replication group must have at least one read replica to enable autofailover. (Service: AmazonElastiCache; Status Code: 400; Error Code: InvalidReplicationGroupState; Request ID: )

这是我的ElastiCache集群：

有人知道这个问题的根本原因是什么吗？