diff --git a/tests/test_inconsistent_data.py b/tests/test_inconsistent_data.py
new file mode 100644
index 00000000..c58087b7
--- /dev/null
+++ b/tests/test_inconsistent_data.py
@@ -0,0 +1,18 @@
+import znh5md
+import numpy.testing as npt
+
+
+def test_keys_missing(tmp_path, s22, s22_energy_forces):
+    io = znh5md.IO(tmp_path / "test.h5")
+
+
+    images = s22_energy_forces + s22
+    io.extend(images)
+    assert len(io) == len(images)
+    assert len(list(io)) == len(images)
+
+    for a, b in zip(images, io):
+        assert a == b
+        b.get_potential_energy()
+        assert a.get_potential_energy() == b.get_potential_energy()
+        npt.assert_array_equal(a.get_forces(), b.get_forces())
diff --git a/znh5md/format.py b/znh5md/format.py
index 677f6ae2..db5a38b8 100644
--- a/znh5md/format.py
+++ b/znh5md/format.py
@@ -288,10 +288,12 @@ def _combine_dicts(dicts: List[Dict[str, np.ndarray]]) -> Dict[str, np.ndarray]:
     """Helper function to combine dictionaries containing numpy arrays."""
     combined = {}
     for key in dicts[0]:
-        combined[key] = concatenate_varying_shape_arrays(
-            [
-                d[key] if isinstance(d[key], np.ndarray) else np.array(d[key])
-                for d in dicts
-            ]
-        )
+        data = []
+        for d in dicts:
+            if key in d:
+                data.append(d[key])
+        if data:
+            combined[key] = concatenate_varying_shape_arrays(data)
+        else:
+            raise ValueError(f"Key {key} is missing in one of the data objects.")
     return combined