Failure when producing block with no transactions - Not producing block because node didn't wake up within 500ms of the slot time.

Hi have configured my steem, based in the article:
https://steemit.com/steemhelp/@pharesim/ubuntu-14-04-miner-setup

To the config file I added the suggested configuration:

  witness = "initminer"

# WIF private key (may specify multiple times)
  private-key = 5JNHxxxxxxxx

# Endpoint for P2P node to listen on
  p2p-endpoint = 0.0.0.0:3333

# Endpoint for websocket RPC to listen on
  rpc-endpoint = 127.0.0.1:9876```

Also, the  miner configuration.
```# name of miner and its private key (e.g. ["account","WIF PRIVATE KEY"] )
miner = ["initminer","5JNHxxxxxxxx"]```

But, when I excecute the ./steemd with

```# ./steemd -d testnet --enable-stale-production 
------------------------------------------------------

            STARTING TEST NETWORK

------------------------------------------------------
initminer public key: TST6xxxxxxxxxxxxxxxxx
initminer private key: 5JNHxxxxxxxxxxxxxxxx
chain id: 18dcfxxxxxxxxxxxxxxxxxxxxxxxxxxxx
------------------------------------------------------
3594863ms th_a       witness.cpp:101               plugin_initialize    ] _witnesses: ["initminer"] 
3594878ms th_a       thread.cpp:95                 thread               ] name:ntp tid:139692147705600
3594880ms th_a       thread.cpp:95                 thread               ] name:p2p tid:139691798034176
3594908ms ntp        ntp.cpp:177                   read_loop            ] ntp_delta_time updated to 25484061 us
3595000ms th_a       witness.cpp:283               block_production_loo ] Not producing block because node didn't wake up within 500ms of the slot time.


43517ms th_a       database.cpp:2681             _apply_block         ] 10 assert_exception: Assert Exception
active_witnesses.size() == STEEMIT_MAX_MINERS: number of active witnesses does not equal STEEMIT_MAX_MINERS
    {"active_witnesses.size()":1,"STEEMIT_MAX_MINERS":21}
    th_a  database.cpp:1128 update_witness_schedule4
43517ms th_a       database.cpp:637              _push_block          ] Failed to push new block:
10 assert_exception: Assert Exception
active_witnesses.size() == STEEMIT_MAX_MINERS: number of active witnesses does not equal STEEMIT_MAX_MINERS
    {"active_witnesses.size()":1,"STEEMIT_MAX_MINERS":21}
    th_a  database.cpp:1128 update_witness_schedule4
rethrow
    {}
    th_a  database.cpp:2681 _apply_block
43517ms th_a       witness.cpp:385               maybe_produce_block  ] 10 assert_exception: Assert Exception
active_witnesses.size() == STEEMIT_MAX_MINERS: number of active witnesses does not equal STEEMIT_MAX_MINERS
    {"active_witnesses.size()":1,"STEEMIT_MAX_MINERS":21}
    th_a  database.cpp:1128 update_witness_schedule4
rethrow
    {}
    th_a  database.cpp:2681 _apply_block

    {"new_block":{"previous":"0000001407585ff1653019e557699e032f387984","timestamp":"2016-08-26T05:01:09","witness":"initminer","transaction_merkle_root":"0000000000000000000000000000000000000000","extensions":[],"witness_signature":"1f0f2cf9411eefc5d16e7a989889a8fd1a2d8f8c49fd60bc9387394c7996195bea35645c188e83b16701d8c1861321a973481b391d31d550eb849117839fe4c829","transactions":[]}}
    th_a  database.cpp:556 operator()

    {"witness_owner":"initminer"}
    th_a  database.cpp:712 operator()
43517ms th_a       witness.cpp:386               maybe_produce_block  ] Clearing pending transactions and attempting again
43519ms th_a       database.cpp:2681             _apply_block         ] 10 assert_exception: Assert Exception
active_witnesses.size() == STEEMIT_MAX_MINERS: number of active witnesses does not equal STEEMIT_MAX_MINERS
    {"active_witnesses.size()":1,"STEEMIT_MAX_MINERS":21}
    th_a  database.cpp:1128 update_witness_schedule4
43519ms th_a       database.cpp:637              _push_block          ] Failed to push new block:
10 assert_exception: Assert Exception
active_witnesses.size() == STEEMIT_MAX_MINERS: number of active witnesses does not equal STEEMIT_MAX_MINERS
    {"active_witnesses.size()":1,"STEEMIT_MAX_MINERS":21}
    th_a  database.cpp:1128 update_witness_schedule4
rethrow
    {}
    th_a  database.cpp:2681 _apply_block
43519ms th_a       witness.cpp:385               maybe_produce_block  ] 10 assert_exception: Assert Exception
active_witnesses.size() == STEEMIT_MAX_MINERS: number of active witnesses does not equal STEEMIT_MAX_MINERS
    {"active_witnesses.size()":1,"STEEMIT_MAX_MINERS":21}
    th_a  database.cpp:1128 update_witness_schedule4
rethrow
    {}
    th_a  database.cpp:2681 _apply_block

    {"new_block":{"previous":"0000001407585ff1653019e557699e032f387984","timestamp":"2016-08-26T05:01:09","witness":"initminer","transaction_merkle_root":"0000000000000000000000000000000000000000","extensions":[],"witness_signature":"1f0f2cf9411eefc5d16e7a989889a8fd1a2d8f8c49fd60bc9387394c7996195bea35645c188e83b16701d8c1861321a973481b391d31d550eb849117839fe4c829","transactions":[]}}
    th_a  database.cpp:556 operator()

    {"witness_owner":"initminer"}
    th_a  database.cpp:712 operator()
43519ms th_a       witness.cpp:386               maybe_produce_block  ] Clearing pending transactions and attempting again
43519ms th_a       witness.cpp:289               block_production_loo ] Failure when producing block with no transactions
44516ms th_a       witness.cpp:283               block_production_loo ] Not producing block because node didn't wake up within 500ms of the slot time.
45516ms th_a       witness.cpp:283               block_production_loo ] Not producing block because node didn't wake up within 500ms of the slot time.
46516ms th_a       database.cpp:2681             _apply_block         ] 10 assert_exception: Assert Exception
active_witnesses.size() == STEEMIT_MAX_MINERS: number of active witnesses does not equal STEEMIT_MAX_MINERS
    {"active_witnesses.size()":1,"STEEMIT_MAX_MINERS":21}