diff --git a/flox/core.py b/flox/core.py
index 42bc2f1a2..8edd7c22b 100644
--- a/flox/core.py
+++ b/flox/core.py
@@ -1507,17 +1507,15 @@ def dask_groupby_agg(
     group_chunks: tuple[tuple[int | float, ...]]
 
     if method in ["map-reduce", "cohorts"]:
-        combine: Callable[..., IntermediateDict]
-        if do_simple_combine:
-            combine = partial(_simple_combine, reindex=reindex)
-            combine_name = "simple-combine"
-        else:
-            combine = partial(_grouped_combine, engine=engine, sort=sort)
-            combine_name = "grouped-combine"
+        combine: Callable[..., IntermediateDict] = (
+            partial(_simple_combine, reindex=reindex)
+            if do_simple_combine
+            else partial(_grouped_combine, engine=engine, sort=sort)
+        )
 
         tree_reduce = partial(
             dask.array.reductions._tree_reduce,
-            name=f"{name}-reduce-{method}-{combine_name}",
+            name=f"{name}-reduce-{method}",
             dtype=array.dtype,
             axis=axis,
             keepdims=True,
diff --git a/tests/test_core.py b/tests/test_core.py
index 880284b75..1ee24550f 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -4,12 +4,14 @@
 import warnings
 from functools import partial, reduce
 from typing import TYPE_CHECKING, Callable
+from unittest.mock import MagicMock, patch
 
 import numpy as np
 import pandas as pd
 import pytest
 from numpy_groupies.aggregate_numpy import aggregate
 
+import flox
 from flox import xrutils
 from flox.aggregations import Aggregation, _initialize_aggregation
 from flox.core import (
@@ -303,6 +305,7 @@ def test_groupby_reduce_all(nby, size, chunks, func, add_nan_by, engine):
         if chunks == -1:
             params.extend([("blockwise", None)])
 
+        combine_error = RuntimeError("This combine should not have been called.")
         for method, reindex in params:
             call = partial(
                 groupby_reduce, array, *by, method=method, reindex=reindex, **flox_kwargs
@@ -312,13 +315,22 @@ def test_groupby_reduce_all(nby, size, chunks, func, add_nan_by, engine):
                 with pytest.raises(NotImplementedError):
                     call()
                 continue
-            actual, *groups = call()
-            if method != "blockwise":
+
+            if method == "blockwise":
+                # no combine necessary
+                mocks = {
+                    "_simple_combine": MagicMock(side_effect=combine_error),
+                    "_grouped_combine": MagicMock(side_effect=combine_error),
+                }
+            else:
                 if "arg" not in func:
                     # make sure we use simple combine
-                    assert any("simple-combine" in key for key in actual.dask.layers.keys())
+                    mocks = {"_grouped_combine": MagicMock(side_effect=combine_error)}
                 else:
-                    assert any("grouped-combine" in key for key in actual.dask.layers.keys())
+                    mocks = {"_simple_combine": MagicMock(side_effect=combine_error)}
+
+            with patch.multiple(flox.core, **mocks):
+                actual, *groups = call()
             for actual_group, expect in zip(groups, expected_groups):
                 assert_equal(actual_group, expect, tolerance)
             if "arg" in func: