Reject negative unit-cell lengths in Timestep

package/MDAnalysis/coordinates/timestep.pyx

@@ -468,6 +468,11 @@ cdef class Timestep:
         if new_dimensions is None:
             self._unitcell[:] = 0
         else:
+            new_dimensions = np.asarray(new_dimensions, dtype=np.float32)
+            if new_dimensions.ndim == 0 and new_dimensions < 0:
+                raise ValueError("Unit cell lengths must be non-negative")
+            if new_dimensions.size == 6 and (new_dimensions.reshape(-1)[:3] < 0).any():
+                raise ValueError("Unit cell lengths must be non-negative")
             self._unitcell[:] = new_dimensions
 
     @property

testsuite/MDAnalysisTests/coordinates/test_timestep_api.py

@@ -828,6 +828,17 @@ def test_set_zero_box(self, ts):
         assert ts.dimensions is None
         assert ts.triclinic_dimenions is None
 
+    @pytest.mark.parametrize("value", [-1.0, -np.inf])
+    def test_set_negative_length_rejected(self, ts, value):
+        with pytest.raises(ValueError, match="non-negative"):
+            ts.dimensions = [value, 2, 3, 90, 90, 90]
+
+    def test_scalar_dimensions_broadcast_is_preserved(self, ts):
+        ts.dimensions = 7
+        assert np.all(ts.dimensions == 7)
+        with pytest.raises(ValueError, match="non-negative"):
+            ts.dimensions = -1
+
     def test_set_zero_box_triclinic(self, ts):
         ts.triclinic_dimensions = np.zeros((3, 3))