Why is GetBranchInput seeing extended alpha fields as blank?

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I'm getting the following error in a fuel cell CHP model which has multiple SOFC modules in series on a single branch. It only seems to occur when there are 11 components on the branch. It works fine with six components in series but the error doesn't mention the number of components. It looks to me like extended alpha fields 3 and 4 contain "SOFC Heat Rec Inlet Node 11" and "SOFC Heat Rec Outlet Node" and so why is EnergyPlus seeing them as blank?

** Severe  ** GetBranchInput: Branch="FUEL_CELL_CHP PRIMARY PLANT LOOP BRANCH", invalid data.
**   ~~~   ** blank required field: Extended Alpha Field 3
**   ~~~   ** Occurs on Extended Alpha Field 1="GENERATOR:FUELCELL:EXHAUSTGASTOWATERHEATEXCHANGER", Extended Alpha Field 2="PRIMARYHEATER 11".
** Severe  ** GetBranchInput: Branch="FUEL_CELL_CHP PRIMARY PLANT LOOP BRANCH", invalid data.
**   ~~~   ** blank required field: Extended Alpha Field 4
**   ~~~   ** Occurs on Extended Alpha Field 1="GENERATOR:FUELCELL:EXHAUSTGASTOWATERHEATEXCHANGER", Extended Alpha Field 2="PRIMARYHEATER 11".
** Severe  ** GetBranchInput:  Invalid Branch Input, preceding condition(s) will likely cause termination.

The branch is as follows:

Branch,
Fuel_Cell_CHP Primary Plant Loop Branch,
autosize,
,
Generator:FuelCell:ExhaustGasToWaterHeatExchanger,
PrimaryHeater 0,
SOFC Heat Rec Inlet Node,
SOFC Heat Rec Inlet Node 1,
Active,
Generator:FuelCell:ExhaustGasToWaterHeatExchanger,
PrimaryHeater 1,
SOFC Heat Rec Inlet Node 1,
SOFC Heat Rec Inlet Node 2,
Active,
    <snipped>
Generator:FuelCell:ExhaustGasToWaterHeatExchanger,
PrimaryHeater 10,
SOFC Heat Rec Inlet Node 10,
SOFC Heat Rec Inlet Node 11,
Active,
Generator:FuelCell:ExhaustGasToWaterHeatExchanger,
PrimaryHeater 11,
SOFC Heat Rec Inlet Node 11,
SOFC Heat Rec Outlet Node,
Active ;