diff --git a/src/spikeinterface/core/base.py b/src/spikeinterface/core/base.py
index ba18cf09b6..ad31b97d8e 100644
--- a/src/spikeinterface/core/base.py
+++ b/src/spikeinterface/core/base.py
@@ -47,6 +47,10 @@ def __init__(self, main_ids: Sequence) -> None:
         # 'main_ids' will either be channel_ids or units_ids
         # They are used for properties
         self._main_ids = np.array(main_ids)
+        if len(self._main_ids) > 0:
+            assert (
+                self._main_ids.dtype.kind in "uiSU"
+            ), f"Main IDs can only be integers (signed/unsigned) or strings, not {self._main_ids.dtype}"
 
         # dict at object level
         self._annotations = {}
diff --git a/src/spikeinterface/core/baserecordingsnippets.py b/src/spikeinterface/core/baserecordingsnippets.py
index affde8a75e..d411f38d2a 100644
--- a/src/spikeinterface/core/baserecordingsnippets.py
+++ b/src/spikeinterface/core/baserecordingsnippets.py
@@ -1,4 +1,4 @@
-from typing import List
+from __future__ import annotations
 from pathlib import Path
 
 import numpy as np
@@ -19,7 +19,7 @@ class BaseRecordingSnippets(BaseExtractor):
 
     has_default_locations = False
 
-    def __init__(self, sampling_frequency: float, channel_ids: List, dtype):
+    def __init__(self, sampling_frequency: float, channel_ids: list[str, int], dtype: np.dtype):
         BaseExtractor.__init__(self, channel_ids)
         self._sampling_frequency = sampling_frequency
         self._dtype = np.dtype(dtype)
diff --git a/src/spikeinterface/core/basesnippets.py b/src/spikeinterface/core/basesnippets.py
index f35bc2b266..b4e3c11f55 100644
--- a/src/spikeinterface/core/basesnippets.py
+++ b/src/spikeinterface/core/basesnippets.py
@@ -1,10 +1,8 @@
 from typing import List, Union
-from pathlib import Path
 from .base import BaseSegment
 from .baserecordingsnippets import BaseRecordingSnippets
 import numpy as np
 from warnings import warn
-from probeinterface import Probe, ProbeGroup, write_probeinterface, read_probeinterface, select_axes
 
 # snippets segments?
 
diff --git a/src/spikeinterface/core/npysnippetsextractor.py b/src/spikeinterface/core/npysnippetsextractor.py
index 80979ce6c9..69c48356e5 100644
--- a/src/spikeinterface/core/npysnippetsextractor.py
+++ b/src/spikeinterface/core/npysnippetsextractor.py
@@ -27,6 +27,9 @@ def __init__(
         num_segments = len(file_paths)
         data = np.load(file_paths[0], mmap_mode="r")
 
+        if channel_ids is None:
+            channel_ids = np.arange(data["snippet"].shape[2])
+
         BaseSnippets.__init__(
             self,
             sampling_frequency,
@@ -84,7 +87,7 @@ def write_snippets(snippets, file_paths, dtype=None):
             arr = np.empty(n, dtype=snippets_t, order="F")
             arr["frame"] = snippets.get_frames(segment_index=i)
             arr["snippet"] = snippets.get_snippets(segment_index=i).astype(dtype, copy=False)
-
+            file_paths[i].parent.mkdir(parents=True, exist_ok=True)
             np.save(file_paths[i], arr)
 
 
diff --git a/src/spikeinterface/extractors/cellexplorersortingextractor.py b/src/spikeinterface/extractors/cellexplorersortingextractor.py
index 31241a4147..0980e89f1c 100644
--- a/src/spikeinterface/extractors/cellexplorersortingextractor.py
+++ b/src/spikeinterface/extractors/cellexplorersortingextractor.py
@@ -118,7 +118,7 @@ def __init__(
         spike_times = spikes_data["times"]
 
         # CellExplorer reports spike times in units seconds; SpikeExtractors uses time units of sampling frames
-        unit_ids = unit_ids[:].tolist()
+        unit_ids = [str(unit_id) for unit_id in unit_ids]
         spiketrains_dict = {unit_id: spike_times[index] for index, unit_id in enumerate(unit_ids)}
         for unit_id in unit_ids:
             spiketrains_dict[unit_id] = (sampling_frequency * spiketrains_dict[unit_id]).round().astype(np.int64)