diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index 0bd76e8..ebfea68 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -85,8 +85,16 @@ jobs:
           test -f .coverage || uvx hatch run ${{ matrix.env.name }}:cov-combine
           uvx hatch run ${{ matrix.env.name }}:cov-report   # report visibly
           uvx hatch run ${{ matrix.env.name }}:coverage xml # create report for upload
+          # Quick sanity check
+          grep -m1 'filename=' -n coverage.xml || { echo "coverage.xml missing filenames"; exit 1; }
       - name: Upload coverage
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV }}
         uses: codecov/codecov-action@v5
+        with:
+          token: ${{ secrets.CODECOV }}
+          files: ./coverage.xml
+          verbose: true
 
   # Check that all tests defined above pass. This makes it easy to set a single "required" test in branch
   # protection instead of having to update it frequently. See https://github.com/re-actors/alls-green#why.
diff --git a/README.md b/README.md
index 3ab68d6..82e2a70 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,7 @@
 [![Tests][badge-tests]][link-tests]
 [![Documentation][badge-docs]][link-docs]
 [![PyPI](https://img.shields.io/pypi/v/treedata.svg)](https://pypi.org/project/treedata)
+[![codecov](https://codecov.io/github/YosefLab/treedata/branch/main/graph/badge.svg?token=YPhuxlw4my)](https://codecov.io/github/YosefLab/treedata)
 
 [badge-tests]: https://img.shields.io/github/actions/workflow/status/YosefLab/treedata/test.yaml?branch=main
 [link-tests]: https://github.com/YosefLab/treedata/actions/workflows/test.yaml
diff --git a/src/treedata/_core/treedata.py b/src/treedata/_core/treedata.py
index 4109ae7..b772894 100755
--- a/src/treedata/_core/treedata.py
+++ b/src/treedata/_core/treedata.py
@@ -301,6 +301,12 @@ def vart(self, value):
         self._vart = vart
         self._update_has_overlap()
 
+    @label.setter
+    def label(self, value):
+        if value is not None and not isinstance(value, str):
+            raise ValueError("label has to be a string or None")
+        self._tree_label = value
+
     @allow_overlap.setter
     def allow_overlap(self, value):
         if not isinstance(value, bool):
diff --git a/tests/test_merge.py b/tests/test_merge.py
index a84f638..746aa0e 100755
--- a/tests/test_merge.py
+++ b/tests/test_merge.py
@@ -144,6 +144,18 @@ def test_subset_alignment(nodes_tdata):
     assert list(tdata.vart.keys()) == ["tree"]
 
 
+def test_merge_multiple_values(tdata_list):
+    # Test merging with mumtiple tree values
+    tdata_list[0].label = "other"
+    with pytest.warns(UserWarning):
+        tdata = td.concat(tdata_list, axis=0, join="outer", merge="first")
+    assert tdata.label == "tree"
+    tdata_list[1].alignment = "subset"
+    with pytest.warns(UserWarning):
+        tdata = td.concat(tdata_list, axis=0, join="outer", merge="first")
+    assert tdata.alignment == "subset"
+
+
 def test_concat_bad_index(tdata_list):
     tdata_list[0].obs.index = tdata_list[1].obs.index
     with pytest.raises(ValueError):