diff --git a/sleap_roots/lengths.py b/sleap_roots/lengths.py
index a7938b6..e4af92d 100644
--- a/sleap_roots/lengths.py
+++ b/sleap_roots/lengths.py
@@ -125,7 +125,7 @@ def get_curve_index(
         & (~np.isnan(base_tip_dists))
         & (lengths > 0)
         & (lengths >= base_tip_dists),
-        (lengths - base_tip_dists) / lengths,
+        (lengths - base_tip_dists) / np.where(lengths != 0, lengths, np.nan),
         np.nan,
     )
 
diff --git a/tests/test_lengths.py b/tests/test_lengths.py
index 263f805..004fd61 100644
--- a/tests/test_lengths.py
+++ b/tests/test_lengths.py
@@ -288,7 +288,6 @@ def test_invalid_scalar_values():
     assert np.isnan(get_curve_index(0, 8))
 
 
-# tests for `get_root_lengths`
 def test_curve_index_float():
     assert get_curve_index(10.0, 5.0) == 0.5