Reject invalid unit-cell angles

package/MDAnalysis/coordinates/timestep.pyx

@@ -468,6 +468,18 @@ 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:
+                if new_dimensions != 0:
+                    raise ValueError("Unit cell dimensions must contain six values")
+                self._unitcell[:] = 0
+                return
+            if new_dimensions.size == 6:
+                values = new_dimensions.reshape(-1)
+                if (values[:3] < 0).any():
+                    raise ValueError("Unit cell lengths must be non-negative")
+                if not np.isfinite(values[3:]).all() or not ((values[3:] > 0) & (values[3:] < 180)).all():
+                    raise ValueError("Unit cell angles must be finite and strictly between 0 and 180 degrees")
             self._unitcell[:] = new_dimensions
 
     @property

testsuite/MDAnalysisTests/coordinates/test_timestep_api.py

@@ -170,6 +170,20 @@ def test_dimensions_set_box(self, ts, dtype):
         assert ts.dimensions.dtype == np.float32
         assert_allclose(ts.dimensions, self.newbox)
 
+    def test_dimensions_reject_scalar_broadcast(self, ts):
+        with pytest.raises(ValueError, match="six values"):
+            ts.dimensions = 5
+        ts.dimensions = 0
+        assert ts.dimensions is None
+
+    @pytest.mark.parametrize(
+        "angles",
+        ([0, 90, 90], [180, 90, 90], [-1, 90, 90], [np.nan, 90, 90], [np.inf, 90, 90]),
+    )
+    def test_dimensions_reject_invalid_angles(self, ts, angles):
+        with pytest.raises(ValueError, match="angles"):
+            ts.dimensions = [10, 10, 10, *angles]
+
     def test_volume(self, ts):
         ts.dimensions = self.newbox
         assert_equal(ts.volume, self.ref_volume)