diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index d270069b..220d50d3 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -39,7 +39,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
@@ -49,8 +49,22 @@ jobs:
           cache-dependency-path: "pyproject.toml"
       - run: sudo apt-get install -y libbz2-dev # required to build fitsio
       - run: pip install -c .github/test-constraints.txt '.[test]'
-      - run: pytest --cov=heracles --cov-report=lcov
+      - run: pytest --cov=heracles --cov-report=xml
       - uses: coverallsapp/github-action@v2
+        with:
+          parallel: true
+          flag-name: run-${{ matrix.python-version }}
+
+  finish:
+    name: Finish
+    runs-on: ubuntu-latest
+    needs: test
+    if: always()
+    steps:
+      - uses: coverallsapp/github-action@v2
+        with:
+          parallel-finished: true
+          carryforward: "run-3.9,run-3.10,run-3.11,run-3.12,run-3.13"
 
   build:
     name: Build
diff --git a/heracles/io.py b/heracles/io.py
index 90044a8f..03eed048 100644
--- a/heracles/io.py
+++ b/heracles/io.py
@@ -618,6 +618,8 @@ def __getitem__(self, key):
         data = self._cache.get(ext)
         if data is None:
             with self.fits as fits:
+                if ext not in fits:
+                    raise KeyError(ext)
                 data = self.reader(fits[ext])
             self._cache[ext] = data
         return data
diff --git a/tests/test_io.py b/tests/test_io.py
index 239ab56d..27dc1882 100644
--- a/tests/test_io.py
+++ b/tests/test_io.py
@@ -8,18 +8,30 @@ def zbins():
     return {0: (0.0, 0.8), 1: (1.0, 1.2)}
 
 
+@pytest.fixture
+def zbins2():
+    return {2: (0.0, 0.8), 3: (1.0, 1.2)}
+
+
 @pytest.fixture
 def mock_alms(rng, zbins):
+    return generate_mock_alms(rng, zbins)
+
+
+@pytest.fixture
+def mock_alms2(rng, zbins2):
+    return generate_mock_alms(rng, zbins2)
+
+
+def generate_mock_alms(rng, zbins):
     import numpy as np
 
     lmax = 32
-
     Nlm = (lmax + 1) * (lmax + 2) // 2
 
-    # names and spins
     fields = {"P": 0, "G": 2}
-
     alms = {}
+
     for n, s in fields.items():
         shape = (Nlm, 2) if s == 0 else (2, Nlm, 2)
         for i in zbins:
@@ -258,17 +270,67 @@ def test_write_read_alms(mock_alms, tmp_path):
     import numpy as np
 
     path = tmp_path / "alms.fits"
-
     heracles.write_alms(path, mock_alms)
     assert path.exists()
-    alms = heracles.read_alms(path)
 
+    alms = heracles.read_alms(path)
     assert alms.keys() == mock_alms.keys()
+
     for key in mock_alms:
         np.testing.assert_array_equal(mock_alms[key], alms[key])
         assert mock_alms[key].dtype.metadata == alms[key].dtype.metadata
 
 
+def test_fits_dict_keyerror(mock_alms, tmp_path):
+    from heracles.io import FitsDict
+    from pathlib import Path
+
+    tmp_path = Path(tmp_path)
+    path1 = tmp_path / "alms1.fits"
+
+    heracles.write_alms(path1, mock_alms)
+
+    assert path1.exists()
+
+    alms1 = FitsDict(path1, clobber=False)
+
+    # Ensure bad key raises a key error
+    with pytest.raises(KeyError):
+        _ = alms1["badkey"]
+
+    # Ensure an existing key does NOT raise an error
+    _ = alms1[("P", 0)]
+
+
+def test_chain_almfits(mock_alms, mock_alms2, tmp_path):
+    from heracles.io import AlmFits
+    from collections import ChainMap
+    import numpy as np
+
+    path1 = tmp_path / "alms1.fits"
+    path2 = tmp_path / "alms2.fits"
+
+    heracles.write_alms(path1, mock_alms)
+    heracles.write_alms(path2, mock_alms2)
+
+    assert path1.exists()
+    assert path2.exists()
+
+    alms1 = AlmFits(path1, clobber=False)
+    alms2 = AlmFits(path2, clobber=False)
+    chained_alms = ChainMap()
+
+    chained_alms.maps.append(alms1)
+    chained_alms.maps.append(alms2)
+
+    for key in mock_alms:
+        np.testing.assert_array_equal(mock_alms[key], chained_alms[key])
+        assert mock_alms[key].dtype.metadata == chained_alms[key].dtype.metadata
+    for key in mock_alms2:
+        np.testing.assert_array_equal(mock_alms2[key], chained_alms[key])
+        assert mock_alms2[key].dtype.metadata == chained_alms[key].dtype.metadata
+
+
 def test_write_read_cls(mock_cls, tmp_path):
     import numpy as np
 
diff --git a/tests/test_result.py b/tests/test_result.py
index 3b1935fe..921536a9 100644
--- a/tests/test_result.py
+++ b/tests/test_result.py
@@ -88,7 +88,7 @@ def test_result_2d(rng):
 @pytest.mark.parametrize("weight", [None, "l(l+1)", "2l+1", "<rand>"])
 @pytest.mark.parametrize("ndim,axis", [(1, 0), (2, 0), (3, 1)])
 def test_binned(ndim, axis, weight, rng):
-    shape = rng.integers(0, 100, ndim)
+    shape = rng.integers(1, 100, ndim)
     lmax = shape[axis] - 1
 
     data = heracles.Result(rng.standard_normal(shape), axis=axis)