diff --git a/pyat/at/latticetools/response_matrix.py b/pyat/at/latticetools/response_matrix.py
index 28f12c3a3..d6fbaa186 100644
--- a/pyat/at/latticetools/response_matrix.py
+++ b/pyat/at/latticetools/response_matrix.py
@@ -101,19 +101,18 @@
 from __future__ import annotations
 
 __all__ = [
-    "split",
+    "sequence_split",
     "ResponseMatrix",
     "OrbitResponseMatrix",
     "TrajectoryResponseMatrix",
 ]
 
 import os
-import math
 import copy
 import multiprocessing
 import concurrent.futures
 import abc
-from collections.abc import Sequence
+from collections.abc import Sequence, Generator
 from itertools import chain
 from functools import partial
 
@@ -121,7 +120,7 @@
 
 from .observables import TrajectoryObservable, OrbitObservable, LatticeObservable
 from .observablelist import ObservableList
-from ..lattice import AtError, Lattice, Refpts, Orbit, AxisDef, plane_
+from ..lattice import AtError, Lattice, Refpts, AxisDef, plane_
 from ..lattice import Monitor, checkattr
 from ..lattice.lattice_variables import RefptsVariable
 from ..lattice.variables import VariableList
@@ -132,21 +131,32 @@
 _globobs: ObservableList | None = None
 
 
-def split(ary: Sequence, nslices):
+def sequence_split(seq: Sequence, nslices: int) -> Generator[Sequence, None, None]:
+    """Split a sequence into multiple sub-sequences.
 
-    def _spl(lll):
+    The length of *seq* does not have to be a multiple of *nslices*.
+
+    Args:
+        seq: sequence to split
+        nslices: number of sub-sequences
+
+    Returns:
+        subseqs: Iterator over sub-sequences
+    """
+
+    def _split(seqsizes):
         beg = 0
-        for k in lll:
-            end = beg + k
-            yield ary[beg:end]
+        for size in seqsizes:
+            end = beg + size
+            yield seq[beg:end]
             beg = end
 
-    lna = len(ary)
-    sz  = math.trunc(lna/nslices)
-    lsl = [sz] * nslices
-    for k in range(lna - sz * nslices):
-        lsl[k] += 1
-    return list(_spl(lsl))
+    lna = len(seq)
+    sz, rem = divmod(lna, nslices)
+    lsubseqs = [sz] * nslices
+    for k in range(rem):
+        lsubseqs[k] += 1
+    return _split(lsubseqs)
 
 
 def _resp(
@@ -431,7 +441,7 @@ def build(
             ctx = multiprocessing.get_context(start_method)
             if pool_size is None:
                 pool_size = min(len(self.variables), os.cpu_count())
-            obschunks = np.array_split(self.variables, pool_size)
+            obschunks = sequence_split(self.variables, pool_size)
             if ctx.get_start_method() == "fork":
                 _globring = self.ring
                 _globobs = self.observables