[ase-users] Castep calculator cell_constraints format question

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I am trying to learn how to apply ASE (3.19.1) to CASTEP (19.11) on Mac OS 10.15.4. I have managed to write a program to optimize the Bi primitive cell.
The program seems to work, however, when I try to introduce cell constraints, the program fails.
The correct form for the cell constraints is as below.

%block cell_constraints
 |a| |b| |c|
  α   β   γ
%endblock cell_constraints

I have tried introducing the command 

calc.cell.cell_constrains = '1 1 1 2 2 2'   (This says keep a=b=c and α=β=γ as I have a rhombohedral cell)

but the program fails with the following output:

application called MPI_Abort(MPI_COMM_WORLD, 1) - process 0
Couldn't fetch number of kpoints from dryrun CASTEP file
Found error in input
ERROR: cell_read - CELL_CONSTRAINTS for formatted correctly
Current trace stack:
 cell_read_wrapped
 castep

What is the correct format for cell_constraints?  Thanks for any help in advance.

Note that the internal documentation echos this:

In [7]: calc.cell.cell_constraints?                                                                                                                                                                      
Type:            CastepOption
String form:     Option: cell_constraints[unset](Block, Basic)
File:            ~/opt/anaconda3/lib/python3.7/site-packages/ase/calculators/castep.py
Docstring:      
A list of constraints on the unit cell parameters used in a  geometry optimisation or molecular dynamics simulation.  Example:
%block cell_constraints
1 1 3  ! a=b can vary but tied equal, c free to vary
0 0 0  ! Fix alpha, beta and gamma
%endblock cell_constraints




Below is the program source.

import ase
from ase.calculators import castep
import ase.io.castep
from ase import Atoms
from ase.io import read, write


# define Bi primitive cell
a = 4.81833
alpha = 56.94372

# prepare atom
Bi = Atoms(['Bi','Bi'], positions=[[0.231989,0.231989,0.231989],[0.768011,0.768011,0.768011]], cell = [a,a,a,alpha,alpha,alpha], pbc=[True,True,True], calculator = castep)

calc = ase.calculators.castep.Castep()
directory = 'CASTEP_Bi'

Bi.set_calculator(calc)

# include interface settings in .param file
calc._export_settings = True

# reuse the same directory
calc._directory = directory
calc._rename_existing_dir = False
calc._label = 'Bi_PBE'

# necessary for tasks with changing positions
# such as GeometryOptimization or MolecularDynamics
calc.param.task = 'GeometryOptimization'

# Param settings
calc.param.xc_functional = 'PBE'
calc.param.cut_off_energy = 450
calc.param.fix_occupancy = False
calc.param.elec_energy_tol = 1E-10
calc.param.geom_force_tol = 0.001
calc.param.geom_max_iter = 100
# Prevent CASTEP from writing *wvfn* files
calc.param.num_dump_cycles = 0

# Cell settings
calc.cell.kpoint_mp_grid = '10 10 10'
calc.cell.fix_com = False
calc.cell.fix_all_cell = False
#calc.cell.cell_constraints = '1 1 1 2 2 2'



# Check for correct input
if calc.dryrun_ok():
       print("Potential Energy: %12.6f" % ( Bi.get_potential_energy()))
else:
    print("Found error in input")
    print(calc._error)



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