diff --git a/dascore/core/coords.py b/dascore/core/coords.py
index e2b338506..fa76dcc3e 100644
--- a/dascore/core/coords.py
+++ b/dascore/core/coords.py
@@ -1392,10 +1392,10 @@ def _maybe_unbox_scalar(value):
 
         def _round_ratio(numerator, denominator, digits):
             """Round numerator/denominator, cheaply for scalars."""
+            # Inputs are always scalar-like (multi-element arrays are rejected
+            # by the pd.isnull check above) and rounding python floats is
+            # ~10x faster than numpy scalars, hence the float conversion.
             ratio = _maybe_unbox_scalar(numerator / denominator)
-            if isinstance(ratio, np.ndarray):  # multi-element array inputs
-                return np.round(ratio, digits)
-            # rounding python floats is ~10x faster than numpy scalars.
             return round(float(ratio), digits)
 
         zero = _TD64_ZERO if is_timedelta64(step) else 0
@@ -1520,6 +1520,19 @@ def sort(self, reverse=False) -> tuple[BaseCoord, slice | ArrayLike]:
         out = self._new_grid(new_start, new_step, len(self))
         return out, slice(None, None, -1)
 
+    def _get_zero_step_index(self, value, forward):
+        """
+        Get the index of a value for a coord with a step of 0.
+
+        Every sample of such a coord equals start, so the index is either the
+        first sample or one just outside the coord, which makes the
+        selection degenerate.
+        """
+        start = self.start
+        if forward:  # index of the first sample >= value
+            return 0 if value <= start else len(self)
+        return 0 if value >= start else -1
+
     def _get_index(self, value, forward=True):
         """Get the index corresponding to a value."""
         if (value := self._get_compatible_value(value)) is None:
@@ -1529,9 +1542,15 @@ def _get_index(self, value, forward=True):
             # Scalar fast path; avoids several small-array allocations.
             # Due to float weirdness we need a little bit of a fudge factor.
             # (float() first since rounding numpy scalars is ~10x slower)
-            fraction = round(float((value - start) / step), 10)
-            if not math.isfinite(fraction):  # e.g. a step of 0
-                return None
+            # A step of 0 is handled after the division (python scalars raise,
+            # numpy scalars give inf/nan) since testing a numpy step for
+            # truthiness up front costs ~10x more on this hot path.
+            try:
+                fraction = round(float((value - start) / step), 10)
+            except ZeroDivisionError:
+                return self._get_zero_step_index(value, forward)
+            if not math.isfinite(fraction):
+                return self._get_zero_step_index(value, forward)
             out = math.ceil(fraction) if forward else math.floor(fraction)
             if forward and out < 0:
                 return None
diff --git a/tests/test_core/test_coords.py b/tests/test_core/test_coords.py
index 9256424e3..fee555613 100644
--- a/tests/test_core/test_coords.py
+++ b/tests/test_core/test_coords.py
@@ -1369,6 +1369,35 @@ def test_coord_range_len_1(self):
         assert len(new) == 1
         assert np.all(new.values == np.zeros(0))
 
+    @pytest.mark.parametrize(
+        "start,unit",
+        [
+            (0, 1),
+            (np.float64(0), np.float64(1)),
+            (np.datetime64("2020-01-01", "ns"), np.timedelta64(1, "s")),
+        ],
+    )
+    def test_select_step_of_0(self, start, unit):
+        """All samples of a step of 0 coord equal start; select on that."""
+        step = start - start  # a zero step of the right type.
+        coord = CoordRange(start=start, stop=start, step=step)
+        # numpy scalars warn (rather than raise) on the zero division.
+        with np.errstate(divide="ignore", invalid="ignore"):
+            # a range which contains start keeps the sample.
+            kept, index = coord.select((start - unit, start + unit))
+            # ranges which don't are degenerate.
+            after, _ = coord.select((start + unit, start + 2 * unit))
+            before, _ = coord.select((start - 2 * unit, start - unit))
+        assert kept == coord
+        assert index == slice(None, 1, None)
+        assert after.degenerate and before.degenerate
+
+    def test_zero_dim_array_inputs(self):
+        """Ensure 0d arrays (which aren't unboxed) can init a CoordRange."""
+        coord = CoordRange(start=np.array(0.0), stop=np.array(10.0), step=np.array(1.0))
+        assert len(coord) == 10
+        assert coord.step == 1.0
+
     def test_reversed_coord(self, evenly_sampled_reversed_coord):
         """Ensure reverse sampling works for evenly sampled coord."""
         coord = evenly_sampled_reversed_coord
diff --git a/tests/test_utils/test_io_utils.py b/tests/test_utils/test_io_utils.py
index a2a353ee9..8c1fcf3f5 100644
--- a/tests/test_utils/test_io_utils.py
+++ b/tests/test_utils/test_io_utils.py
@@ -1240,6 +1240,7 @@ def _memory_file(self, name: str) -> UPath:
             fi.write(b"dascore" * 64)
         return path
 
+    @pytest.mark.concurrency
     def test_racing_callers_download_once(self, monkeypatch, run_in_threads):
         """Callers wanting one resource agree on the path and download it once."""
         resource = self._memory_file("shared.bin")
@@ -1256,6 +1257,7 @@ def _counted(path, local_path):
         assert len(downloads) == 1
         assert results[0].exists()
 
+    @pytest.mark.concurrency
     def test_distinct_resources_are_not_serialized(self, monkeypatch, run_in_threads):
         """Unrelated downloads run at once; one global lock would time out here."""
         resources = [self._memory_file(f"file_{i}.bin") for i in range(4)]
@@ -1311,6 +1313,7 @@ def test_reinit_drops_download_locks(self):
 class TestIOResourceManagerConcurrency:
     """One manager hands every caller the same handle per type."""
 
+    @pytest.mark.concurrency
     def test_racing_callers_share_one_handle(self, tmp_path, run_in_threads):
         """get_resource opens each required type exactly once."""
         path = tmp_path / "concurrent_resource.bin"