Output variable unique Key name for EnergyManagementSystem:Sensor

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I am trying to implement thermostatic control of a zone based on PMV values (Fanger Comfort Criteria) using EMS in Energyplus. I add an EnergyManagementSystem:Sensor object as per the example in the InputOutput Reference documentation as follows (except the sensor is called "currentPMV" and I am trying to extract the value for the "Core_mid" zone):

EnergyManagementSystem:Sensor,
currentPMV,              !- Name
Core_mid,                !- Output:Variable or Output:Meter Index Key Name
Zone Thermal Comfort Fanger Model PMV;  !- Output:Variable or Output:Meter Name

I also define the Output:variable as follows:

Output:Variable,Core_mid,Zone Thermal Comfort Fanger Model PMV ,Timestep;

When I try to simulate, I obtain the following Severe error:

   ** Severe  ** Invalid Output:Variable or Output:Meter Index Key Name =CORE_MID
   **   ~~~   ** For Output:Variable or Output:Meter = ZONE THERMAL COMFORT FANGER MODEL PMV
   **   ~~~   ** Entered in EnergyManagementSystem:Sensor=CURRENTPMV
   **   ~~~   ** Unique Key Name not found.

I have checked that "Zone Thermal Comfort Fanger Model PMV" is listed in the .rdd file. I have also checked that "Core_mid" is a valid zone. I have tried changing the field to other zone names and obtain the same error. Does anyone know why "Core_mid" is not being recognized as a unique key name?

Any help is much appreciated.