diff --git a/flox/core.py b/flox/core.py
index 6bd390137..1437e506c 100644
--- a/flox/core.py
+++ b/flox/core.py
@@ -1388,27 +1388,35 @@ def dask_groupby_agg(
     return (result, groups)
 
 
-def _validate_reindex(reindex: bool | None, func, method: T_Method, expected_groups) -> bool | None:
+def _validate_reindex(
+    reindex: bool | None, func, method: T_Method, expected_groups, by_is_dask: bool
+) -> bool:
     if reindex is True:
         if _is_arg_reduction(func):
             raise NotImplementedError
         if method == "blockwise":
             raise NotImplementedError
 
-    if method == "blockwise" or _is_arg_reduction(func):
-        reindex = False
+    if reindex is None:
+        if method == "blockwise" or _is_arg_reduction(func):
+            reindex = False
 
-    if reindex is None and expected_groups is not None:
-        reindex = True
+        elif expected_groups is not None:
+            reindex = True
+
+        elif method in ["split-reduce", "cohorts"]:
+            reindex = True
+
+        elif method == "map-reduce":
+            if expected_groups is None and by_is_dask:
+                reindex = False
+            else:
+                reindex = True
 
     if method in ["split-reduce", "cohorts"] and reindex is False:
         raise NotImplementedError
 
-    if method in ["split-reduce", "cohorts"] and reindex is None:
-        reindex = True
-
-    # TODO: Should reindex be a bool-only at this point? Would've been nice but
-    # None's are relied on after this function as well.
+    assert isinstance(reindex, bool)
     return reindex
 
 
@@ -1597,7 +1605,6 @@ def groupby_reduce(
             "argreductions not supported for engine='flox' yet."
             "Try engine='numpy' or engine='numba' instead."
         )
-    reindex = _validate_reindex(reindex, func, method, expected_groups)
 
     bys = tuple(np.asarray(b) if not is_duck_array(b) else b for b in by)
     nby = len(bys)
@@ -1606,6 +1613,8 @@ def groupby_reduce(
     if method in ["split-reduce", "cohorts"] and by_is_dask:
         raise ValueError(f"method={method!r} can only be used when grouping by numpy arrays.")
 
+    reindex = _validate_reindex(reindex, func, method, expected_groups, by_is_dask)
+
     if not is_duck_array(array):
         array = np.asarray(array)
     is_bool_array = np.issubdtype(array.dtype, bool)
diff --git a/tests/test_core.py b/tests/test_core.py
index e31f11e56..d7cbc4d8d 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from functools import reduce
+from functools import partial, reduce
 from typing import TYPE_CHECKING
 
 import numpy as np
@@ -13,6 +13,7 @@
     _convert_expected_groups_to_index,
     _get_optimal_chunks_for_groups,
     _normalize_indexes,
+    _validate_reindex,
     factorize_,
     find_group_cohorts,
     groupby_reduce,
@@ -221,14 +222,26 @@ def test_groupby_reduce_all(nby, size, chunks, func, add_nan_by, engine):
         if not has_dask:
             continue
         for method in ["map-reduce", "cohorts", "split-reduce"]:
-            if "arg" in func and method != "map-reduce":
-                continue
-            actual, *groups = groupby_reduce(array, *by, method=method, **flox_kwargs)
-            for actual_group, expect in zip(groups, expected_groups):
-                assert_equal(actual_group, expect, tolerance)
-            if "arg" in func:
-                assert actual.dtype.kind == "i"
-            assert_equal(actual, expected, tolerance)
+            if method == "map-reduce":
+                reindexes = [True, False, None]
+            else:
+                reindexes = [None]
+            for reindex in reindexes:
+                call = partial(
+                    groupby_reduce, array, *by, method=method, reindex=reindex, **flox_kwargs
+                )
+                if "arg" in func:
+                    if method != "map-reduce" or reindex is True:
+                        with pytest.raises(NotImplementedError):
+                            call()
+                        continue
+
+                actual, *groups = call()
+                for actual_group, expect in zip(groups, expected_groups):
+                    assert_equal(actual_group, expect, tolerance)
+                if "arg" in func:
+                    assert actual.dtype.kind == "i"
+                assert_equal(actual, expected, tolerance)
 
 
 @requires_dask
@@ -1125,3 +1138,33 @@ def test_subset_block_2d(flatblocks, expectidx):
     subset = subset_to_blocks(array, flatblocks)
     assert len(subset.dask.layers) == 2
     assert_equal(subset, array.compute()[expectidx])
+
+
+@pytest.mark.parametrize("method", ["map-reduce", "cohorts"])
+@pytest.mark.parametrize(
+    "expected, reindex, func, expected_groups, by_is_dask",
+    [
+        # argmax only False
+        [False, None, "argmax", None, False],
+        # True when by is numpy but expected is None
+        [True, None, "sum", None, False],
+        # False when by is dask but expected is None
+        [False, None, "sum", None, True],
+        # if expected_groups then always True
+        [True, None, "sum", [1, 2, 3], False],
+        [True, None, "sum", ([1], [2]), False],
+        [True, None, "sum", ([1], [2]), True],
+        [True, None, "sum", ([1], None), False],
+        [True, None, "sum", ([1], None), True],
+    ],
+)
+def test_validate_reindex(expected, reindex, func, method, expected_groups, by_is_dask):
+    if by_is_dask and method == "cohorts":
+        # This should error elsewhere
+        pytest.skip()
+    call = partial(_validate_reindex, reindex, func, method, expected_groups, by_is_dask)
+    if "arg" in func and method == "cohorts":
+        with pytest.raises(NotImplementedError):
+            call()
+    else:
+        assert call() == expected