diff --git a/lenskit/metrics/topn.py b/lenskit/metrics/topn.py
index ae1b70f3c..7f616c53d 100644
--- a/lenskit/metrics/topn.py
+++ b/lenskit/metrics/topn.py
@@ -300,11 +300,11 @@ def ndcg(recs, truth, discount=np.log2, k=None):
             The maximum list length.
     """
 
-    tpos = truth.index.get_indexer(recs['item'])
-
     if k is not None:
         recs = recs.iloc[:k]
 
+    tpos = truth.index.get_indexer(recs['item'])
+
     if 'rating' in truth.columns:
         i_rates = np.sort(truth.rating.values)[::-1]
         if k is not None:
diff --git a/lkbuild/boot-env.yml b/lkbuild/boot-env.yml
index 892591fbf..e8d9d7fae 100644
--- a/lkbuild/boot-env.yml
+++ b/lkbuild/boot-env.yml
@@ -7,5 +7,5 @@ dependencies:
 - invoke=1
 - requests=2
 - pip
-- conda-lock
+- conda-lock=0.13
 - mamba
diff --git a/pyproject.toml b/pyproject.toml
index 68b0f0bfc..8c625d2ff 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -53,7 +53,7 @@ dev = [
     "flake8 >= 3",
     "coverage >= 5",
     "pytest-cov >= 2.12",
-    "ipython >= 7",
+    "ipython == 7.*",
     "docopt >= 0.6",
     "tqdm >= 4",
     "sphinx-autobuild >= 2021",
@@ -63,6 +63,8 @@ doc = [
     "sphinxcontrib-bibtex >= 2.0",
     "sphinx_rtd_theme >= 0.5",
     "nbsphinx >= 0.8",
+    "ipython == 7.*",
+    "notebook >=6",
 ]
 demo = [
     "notebook >= 6",
diff --git a/tests/test_topn_ndcg.py b/tests/test_topn_ndcg.py
index 5d1601d9a..2fdf9e573 100644
--- a/tests/test_topn_ndcg.py
+++ b/tests/test_topn_ndcg.py
@@ -87,6 +87,14 @@ def test_ndcg_perfect():
     assert ndcg(recs, truth) == approx(1.0)
 
 
+def test_ndcg_perfect_k_short():
+    recs = pd.DataFrame({'item': [2, 3, 1]})
+    truth = pd.DataFrame({'item': [1, 2, 3], 'rating': [3.0, 5.0, 4.0]})
+    truth = truth.set_index('item')
+    assert ndcg(recs, truth, k=2) == approx(1.0)
+    assert ndcg(recs[:2], truth, k=2) == approx(1.0)
+
+
 def test_ndcg_wrong():
     recs = pd.DataFrame({'item': [1, 2]})
     truth = pd.DataFrame({'item': [1, 2, 3], 'rating': [3.0, 5.0, 4.0]})