diff --git a/tempo_embeddings/text/corpus.py b/tempo_embeddings/text/corpus.py
index b960e41..0bf1ba8 100644
--- a/tempo_embeddings/text/corpus.py
+++ b/tempo_embeddings/text/corpus.py
@@ -65,16 +65,18 @@ def __add__(self, other: "Corpus") -> "Corpus":
             logging.info("No UMAP model has been computed.")
             umap = None
 
-        if self.label == other.label:
+        if self._label == other._label:
             label = self.label
-        elif not self.label and not other.label:
+        elif self._label and other._label:
+            label = " + ".join((self.label, other.label))
+        elif not self._label and not other._label:
             label = None
-        elif self.label:
+        elif self._label:
             label = self.label
-        elif other.label:
+        elif other._label:
             label = other.label
         else:
-            label = " + ".join((self.label, other.label))
+            raise AssertionError("Uncovered label combination; this is a bug.")
 
         return Corpus(
             self._passages + other._passages, label=label or None, umap_model=umap
diff --git a/tests/unit/text/test_corpus.py b/tests/unit/text/test_corpus.py
index 4ac037c..20cdb09 100644
--- a/tests/unit/text/test_corpus.py
+++ b/tests/unit/text/test_corpus.py
@@ -35,6 +35,22 @@ def test_add(self, test_passages):
         expected = Corpus(test_passages[:2], None, umap_model=None)
         assert Corpus([test_passages[0]]) + Corpus([test_passages[1]]) == expected
 
+    @pytest.mark.parametrize(
+        "label1, label2, expected",
+        [
+            (None, None, "None"),
+            ("label1", None, "label1"),
+            (None, "label2", "label2"),
+            ("label1", "label2", "label1 + label2"),
+            ("label", "label", "label"),
+        ],
+    )
+    def test_add_label(self, label1, label2, expected):
+        corpus1 = Corpus(label=label1)
+        corpus2 = Corpus(label=label2)
+
+        assert (corpus1 + corpus2).label == expected
+
     def test_add_umap_fitted(self, corpus):
         corpus2 = Corpus([Passage("test {i}") for i in range(5)])