diff --git a/lib/mpl_toolkits/mplot3d/art3d.py b/lib/mpl_toolkits/mplot3d/art3d.py
index 0467d2e96e5e..deb0ca34302c 100644
--- a/lib/mpl_toolkits/mplot3d/art3d.py
+++ b/lib/mpl_toolkits/mplot3d/art3d.py
@@ -1382,6 +1382,7 @@ def _generate_normals(polygons):
         v2 = np.empty((len(polygons), 3))
         for poly_i, ps in enumerate(polygons):
             n = len(ps)
+            ps = np.asarray(ps)
             i1, i2, i3 = 0, n//3, 2*n//3
             v1[poly_i, :] = ps[i1, :] - ps[i2, :]
             v2[poly_i, :] = ps[i2, :] - ps[i3, :]
diff --git a/lib/mpl_toolkits/mplot3d/tests/test_art3d.py b/lib/mpl_toolkits/mplot3d/tests/test_art3d.py
index f4f7067b76bb..174c12608ae9 100644
--- a/lib/mpl_toolkits/mplot3d/tests/test_art3d.py
+++ b/lib/mpl_toolkits/mplot3d/tests/test_art3d.py
@@ -3,7 +3,11 @@
 import matplotlib.pyplot as plt
 
 from matplotlib.backend_bases import MouseEvent
-from mpl_toolkits.mplot3d.art3d import Line3DCollection, _all_points_on_plane
+from mpl_toolkits.mplot3d.art3d import (
+    Line3DCollection,
+    Poly3DCollection,
+    _all_points_on_plane,
+)
 
 
 def test_scatter_3d_projection_conservation():
@@ -85,3 +89,14 @@ def test_all_points_on_plane():
     # All points lie on a plane
     points = np.array([[0, 0, 0], [0, 1, 0], [1, 0, 0], [1, 1, 0], [1, 2, 0]])
     assert _all_points_on_plane(*points.T)
+
+
+def test_generate_normals():
+    # Smoke test for https://github.com/matplotlib/matplotlib/issues/29156
+    vertices = ((0, 0, 0), (0, 5, 0), (5, 5, 0), (5, 0, 0))
+    shape = Poly3DCollection([vertices], edgecolors='r', shade=True)
+
+    fig = plt.figure()
+    ax = fig.add_subplot(projection='3d')
+    ax.add_collection3d(shape)
+    plt.draw()