Numpy error when converting array of ctypes types to void pointer

I'd like to send a list of strings to a C function:

import ctypes
from ctypes import c_double, c_void_p, Structure, cast, c_size_t, POINTER
import numpy as np

class FFIArray(Structure):
    """
    Convert sequence of structs or types to C-compatible void array

    """
    _fields_ = [("data", c_void_p),
                ("len", c_size_t)]

    @classmethod
    def from_param(cls, seq):
        """  Allow implicit conversions """
        return seq if isinstance(seq, cls) else cls(seq)

    def __init__(self, seq, data_type):
        array = np.ctypeslib.as_array((data_type * len(seq))(*seq))
        self._buffer = array.data
        self.data = cast(
            array.ctypes.data_as(POINTER(data_type)),
            c_void_p
        )
        self.len = len(array)

This works for e.g. c_double, but fails when I try it with c_char_p, with the following error:

ValueError: '<z' is not a valid PEP 3118 buffer format string

Is there a better way to do this? I'm also not wedded to using numpy, although it's useful for converting iterables of numeric types and numpy arrays to _FFIArray elsewhere.

from Numpy error when converting array of ctypes types to void pointer