pint.pint_matrix.combine_design_matrices_by_param throws wrong exception when duplicate parameters are present.

[abhisrkckl]

Here is how to reproduce the issue:

import pint, pint.models
import astropy.units as u
from pint.pint_matrix import DesignMatrixMaker, combine_design_matrices_by_param

parfile = pint.config.examplefile("B1855+09_NANOGrav_dfg+12_TAI.par")
timfile = pint.config.examplefile("B1855+09_NANOGrav_dfg+12.tim")
model, toas = pint.models.get_model_and_toas(parfile, timfile)

dmm = DesignMatrixMaker('phase', u.Quantity(1,''))
dm1 = dmm(toas, model, ['F0'], offset=True)
dm2 = dmm(toas, model, ['F1'], offset=True)

dm12 = combine_design_matrices_by_param(dm1, dm2)

The expected error message is "Input design matrix {} has duplicated parameters with matrix {}".
The actual error message I get is the following. There seems to be an undefined variable "ii" used in the raise statement.

NameError Traceback (most recent call last)
Input In [31], in <cell line: 1>()
----> 1 dm12 = combine_design_matrices_by_param(dm1, dm2)

File ~/anaconda3/envs/pint-devel/lib/python3.9/site-packages/pint_pulsar-0.7+2686.gbfa227ef.dirty-py3.9.egg/pint/pint_matrix.py:611, in combine_design_matrices_by_param(matrix1, matrix2, padding)
607 for d_param in matrix2.derivative_params:
608 if d_param in base_params:
609 raise ValueError(
610 "Input design matrix {} has duplicated "
--> 611 "parameters with matrix {}".format(ii, 0)
612 )
613 # check if input design matrix has same quantity and padding.
614 new_quantity_index = {}

NameError: name 'ii' is not defined