Getting 'unsupported array index type unicode_type' error when selecting a column based on condition in Numba with NumPy structured array

I am trying to select a column of a structured NumPy array. The column to be selected depends on a condition that will be passed to the function. Numba throws the error below when I try to select the column name based on the condition. Otherwise everything works well.

Basic example:
This works:

import numpy as np
import numba

@numba.njit(fastmath=True, cache=True)
def fun(a, b=0):
    c = 'name'
    #if b:
    #    c = 'age'
    return a[c]

a = np.array([('Rex', 9, 81.0), ('Fido', 3, 27.0)],
             dtype=[('name', 'U10'), ('age', 'i4'), ('weight', 'f4')])
             
fun(a)

This doesn't work:

import numpy as np
import numba

@numba.njit(fastmath=True, cache=True)
def fun(a, b=0):
    c = 'name'
    if b:
        c = 'age'
    return a[c]

a = np.array([('Rex', 9, 81.0), ('Fido', 3, 27.0)],
             dtype=[('name', 'U10'), ('age', 'i4'), ('weight', 'f4')])
             
fun(a)

The error:

---------------------------------------------------------------------------
TypingError                               Traceback (most recent call last)
c:\Users\User\Workspaces\temp.ipynb Cell 9 in ()
8     return a[c]
10 a = np.array([('Rex', 9, 81.0), ('Fido', 3, 27.0)],
11              dtype=[('name', 'U10'), ('age', 'i4'), ('weight', 'f4')])
---> 13 fun(a)
File ~\AppData\Roaming\Python\Python38\site-packages\numba\core\dispatcher.py:468, in _DispatcherBase._compile_for_args(self, *args, **kws)
464         msg = (f"{str(e).rstrip()} \n\nThis error may have been caused "
465                f"by the following argument(s):\n{args_str}\n")
466         e.patch_message(msg)
--> 468     error_rewrite(e, 'typing')
469 except errors.UnsupportedError as e:
470     # Something unsupported is present in the user code, add help info
471     error_rewrite(e, 'unsupported_error')
File ~\AppData\Roaming\Python\Python38\site-packages\numba\core\dispatcher.py:409, in _DispatcherBase._compile_for_args..error_rewrite(e, issue_type)
407     raise e
408 else:
--> 409     raise e.with_traceback(None)
TypingError: Failed in nopython mode pipeline (step: nopython frontend)
No implementation of function Function() found for signature:
>>> getitem(unaligned array(Record(name[type=[unichr x 10];offset=0],age[type=int32;offset=40],weight[type=float32;offset=44];48;False), 1d, C), unicode_type)
There are 22 candidate implementations:
- Of which 20 did not match due to:
Overload of function 'getitem': File: : Line N/A.
With argument(s): '(unaligned array(Record(name[type=[unichr x 10];offset=0],age[type=int32;offset=40],weight[type=float32;offset=44];48;False), 1d, C), unicode_type)':
No match.
- Of which 2 did not match due to:
Overload in function 'GetItemBuffer.generic': File: numba\core\typing\arraydecl.py: Line 166.
With argument(s): '(unaligned array(Record(name[type=[unichr x 10];offset=0],age[type=int32;offset=40],weight[type=float32;offset=44];48;False), 1d, C), unicode_type)':
Rejected as the implementation raised a specific error:
NumbaTypeError: unsupported array index type unicode_type in [unicode_type]
raised from C:\Users\User\AppData\Roaming\Python\Python38\site-packages\numba\core\typing\arraydecl.py:72
During: typing of intrinsic-call at C:\Users\User\AppData\Local\Temp\ipykernel_37200\1621110578.py (8)
File "..\..\..\..\..\AppData\Local\Temp\ipykernel_37200\1621110578.py", line 8:

Looks like it's not happy/does not support when the array index is unknown at compile time.

Do you need a solution beyond this example? Otherwise

@numba.njit(fastmath=True, cache=True)
def fun(a, b=0):
    if b:
         return a['name']
    return a['age']

should do