diff --git a/tests/unit/test_ask_tell_optimization.py b/tests/unit/test_ask_tell_optimization.py
index a68d2e4ff6..e75c777c38 100644
--- a/tests/unit/test_ask_tell_optimization.py
+++ b/tests/unit/test_ask_tell_optimization.py
@@ -945,6 +945,7 @@ def test_ask_tell_optimizer_dataset_len_variables(
         dataset = init_dataset
 
     assert AskTellOptimizer.dataset_len({"tag": dataset}) == 2
+    assert AskTellOptimizer.dataset_len({"tag1": dataset, "tag2": dataset}) == 2
 
 
 def test_ask_tell_optimizer_dataset_len_raises_on_inconsistently_sized_datasets(
diff --git a/trieste/ask_tell_optimization.py b/trieste/ask_tell_optimization.py
index f3b9977144..e5e4579c0a 100644
--- a/trieste/ask_tell_optimization.py
+++ b/trieste/ask_tell_optimization.py
@@ -442,8 +442,8 @@ def dataset_len(cls, datasets: Mapping[Tag, Dataset]) -> int:
             for tag, dataset in datasets.items()
             if not LocalizedTag.from_tag(tag).is_local
         ]
-        unique_lens, unique_idxs = tf.unique(dataset_lens)
-        if len(unique_idxs) == 1:
+        unique_lens, _ = tf.unique(dataset_lens)
+        if len(unique_lens) == 1:
             return int(unique_lens[0])
         else:
             raise ValueError(f"Expected unique global dataset size, got {unique_lens}")