diff --git a/python/chemiscope/structures/_ase.py b/python/chemiscope/structures/_ase.py
index 2f9ab5be8..4e00152e7 100644
--- a/python/chemiscope/structures/_ase.py
+++ b/python/chemiscope/structures/_ase.py
@@ -322,7 +322,7 @@ def extract_vectors_from_ase(frames, key="forces", **kwargs):
 
     for f in frames:
         if key not in f.arrays:
-            raise IndexError(f"Key {key} not found in Atoms frame")
+            raise IndexError(f"Key {key} not found in `Atoms.arrays`")
         values = f.arrays[key]
         if len(values.shape) != 2 or values.shape[1] != 3:
             raise ValueError(
@@ -352,7 +352,7 @@ def extract_tensors_from_ase(frames, key="tensor", **kwargs):
 
     for f in frames:
         if key not in f.arrays:
-            raise IndexError(f"Key {key} not found in Atoms frame")
+            raise IndexError(f"Key {key} not found in `Atoms.arrays`")
         values = f.arrays[key]
         if len(values.shape) != 2 or (values.shape[1] != 6 and values.shape[1] != 9):
             raise ValueError(
diff --git a/python/chemiscope/structures/_shapes.py b/python/chemiscope/structures/_shapes.py
index ae06c1957..c79c005e2 100644
--- a/python/chemiscope/structures/_shapes.py
+++ b/python/chemiscope/structures/_shapes.py
@@ -129,13 +129,11 @@ def ellipsoid_from_tensor(tensor, scale=1.0, force_positive=False):
         ax = np.sqrt(eigenvalues[0])
         ay = np.sqrt(eigenvalues[1])
         az = np.sqrt(eigenvalues[2])
-    except FloatingPointError:
-        print(
-            "Non-positive definite tensor found with eigenvalues ",
-            eigenvalues,
-            ".\nIf this is acceptable, set `force_positive=True` to take the absolute values.",
-        )
-        raise
+    except FloatingPointError as e:
+        raise ValueError(
+            f"Non-positive definite tensor found with eigenvalues {eigenvalues}.\n"
+            "If this is acceptable, set `force_positive=True` to take the absolute values.",
+        ) from e
     np.seterr(**old_settings)
 
     # makes sure the rotation is proper