diff --git a/src/pygama/evt/tcm.py b/src/pygama/evt/tcm.py
index fea54d167..ad87d7a47 100644
--- a/src/pygama/evt/tcm.py
+++ b/src/pygama/evt/tcm.py
@@ -81,10 +81,10 @@ def generate_tcm_cols(
     for ii, array in enumerate(coin_data):
         array = np.array(array)
         array_id = array_ids[ii] if array_ids is not None else ii
-        array_id = np.full_like(array, array_id)
+        array_id = np.full_like(array, array_id, dtype=int)
         col_dict = {"array_id": array_id, "coin_data": array}
         if array_idxs is not None:
-            col_dict["array_idx"] = array_idxs[ii]
+            col_dict["array_idx"] = array_idxs.astype(int)[ii]
         dfs.append(pd.DataFrame(col_dict, copy=False))  # don't copy the data!
 
     # concat and sort
diff --git a/tests/evt/test_build_tcm.py b/tests/evt/test_build_tcm.py
new file mode 100644
index 000000000..505196825
--- /dev/null
+++ b/tests/evt/test_build_tcm.py
@@ -0,0 +1,72 @@
+import os
+
+import lgdo
+import numpy as np
+from lgdo import LH5Store
+
+from pygama import evt
+
+
+def test_generate_tcm_cols(lgnd_test_data):
+    f_raw = lgnd_test_data.get_path(
+        "lh5/prod-ref-l200/generated/tier/raw/cal/p03/r001/l200-p03-r001-cal-20230318T012144Z-tier_raw.lh5"
+    )
+    tables = lgdo.ls(f_raw)
+    store = LH5Store()
+    coin_data = []
+    for tbl in tables:
+        ts, _ = store.read_object(f"{tbl}/raw/timestamp", f_raw)
+        coin_data.append(ts)
+
+    tcm_cols = evt.generate_tcm_cols(
+        coin_data, 0, "last", [int(tb[2:]) for tb in tables]
+    )
+    assert isinstance(tcm_cols, dict)
+    for v in tcm_cols.values():
+        assert np.issubdtype(v.dtype, np.integer)
+
+    # fmt: off
+    assert np.array_equal(
+        tcm_cols["cumulative_length"],
+        [
+            1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 20,
+            21, 22, 23, 24, 25, 26, 27, 28, 29, 30,
+        ],
+    )
+    assert np.array_equal(
+        tcm_cols["array_id"],
+        [
+            1084804, 1084803, 1121600, 1084804, 1121600, 1084804, 1121600,
+            1084804, 1084804, 1084804, 1084803, 1084804, 1084804, 1121600,
+            1121600, 1084804, 1121600, 1084804, 1121600, 1084803, 1084803,
+            1121600, 1121600, 1121600, 1084803, 1084803, 1084803, 1084803,
+            1084803, 1084803,
+        ],
+    )
+    assert np.array_equal(
+        tcm_cols["array_idx"],
+        [
+            0, 0, 0, 1, 1, 2, 2, 3, 4, 5, 1, 6, 7, 3, 4, 8, 5, 9, 6, 2, 3, 7,
+            8, 9, 4, 5, 6, 7, 8, 9,
+        ],
+    )
+    # fmt: on
+
+
+def test_build_tcm(lgnd_test_data, tmptestdir):
+    f_raw = lgnd_test_data.get_path(
+        "lh5/prod-ref-l200/generated/tier/raw/cal/p03/r001/l200-p03-r001-cal-20230318T012144Z-tier_raw.lh5"
+    )
+    out_file = f"{tmptestdir}/pygama-test-tcm.lh5"
+    evt.build_tcm(
+        [(f_raw, ["ch1084803/raw", "ch1084804/raw", "ch1121600/raw"])],
+        "timestamp",
+        out_file=out_file,
+        out_name="hardware_tcm",
+        wo_mode="of",
+    )
+    assert os.path.exists(out_file)
+    store = LH5Store()
+    obj, n_rows = store.read_object("hardware_tcm", out_file)
+    assert isinstance(obj, lgdo.Struct)
+    assert list(obj.keys()) == ["cumulative_length", "array_id", "array_idx"]