diff --git a/src/integrators/aov.cpp b/src/integrators/aov.cpp
index b871ae36ae..0c52e3b237 100644
--- a/src/integrators/aov.cpp
+++ b/src/integrators/aov.cpp
@@ -339,7 +339,19 @@ class AOVIntegrator final : public SamplingIntegrator<Float, Spectrum> {
                         else
                             *aovs++ = Float(it->second);
                     } else {
-                        *aovs++ = Float(dr::reinterpret_array<UInt32>(si.shape));
+                        ShapePtr target = dr::select(
+                            si.instance != dr::zeros<ShapePtr>(),
+                            si.instance, si.shape);
+
+                        UInt32 shape_idx = dr::zeros<UInt32>();
+                        uint32_t counter = 1;
+                        for (const ref<Shape> &shape : shapes) {
+                            shape_idx = dr::select(
+                                target == ShapePtr(shape.get()),
+                                UInt32(counter), shape_idx);
+                            ++counter;
+                        }
+                        *aovs++ = Float(shape_idx);
                     }
                     break;
 
diff --git a/src/integrators/tests/test_aov.py b/src/integrators/tests/test_aov.py
index 4e901362af..570f3bfcfb 100644
--- a/src/integrators/tests/test_aov.py
+++ b/src/integrators/tests/test_aov.py
@@ -328,3 +328,67 @@ def test07_test_aov_normalmap(variants_all_ad_rgb):
     assert(dr.allclose(
         image[:,:,3:6].array,
         dr.tile(n, image.shape[0] * image.shape[1])))
+
+
+def test08_shape_index_correct(variants_all_rgb):
+    W, H = 32, 32
+
+    def make_diffuse(r, g, b):
+        return {'type': 'diffuse', 'reflectance': {'type': 'rgb', 'value': [r, g, b]}}
+
+    # Render with box filter + 1spp to get exact per pixel values
+    # and avoid fractional blending.
+    scene = mi.load_dict({
+        'type': 'scene',
+        'sensor': {
+            'type': 'orthographic',
+            'to_world': mi.ScalarTransform4f().look_at(
+                origin=(0, 0, 2),
+                target=(0, 0, 0),
+                up=(0, 1, 0),
+            ),
+            'sampler': {'type': 'independent'},
+            'film': {
+                'type': 'hdrfilm',
+                'width': W, 'height': H,
+                'rfilter': {'type': 'box'},
+            },
+        },
+        'emitter': {
+            'type': 'constant',
+            'radiance': {'type': 'rgb', 'value': 1.0},
+        },
+        'left': {
+            'type': 'rectangle',
+            'material': make_diffuse(1, 0, 0),
+            'to_world': mi.ScalarTransform4f()
+                .translate([-0.5, 0, 0])
+                .scale([0.5, 1.0, 1.0]),
+        },
+        'right': {
+            'type': 'rectangle',
+            'material': make_diffuse(0, 0, 1),
+            'to_world': mi.ScalarTransform4f()
+                .translate([0.5, 0, 0])
+                .scale([0.5, 1.0, 1.0]),
+        },
+    })
+
+    aov_integrator = mi.load_dict({
+        'type': 'aov',
+        'aovs': 'si:shape_index',
+        'my_image': {'type': 'path', 'max_depth': 1},
+    })
+
+    image = aov_integrator.render(scene, seed=0, spp=1)
+
+    # `shape_index` is the last channel (inner RGBA + shape_index = 5 channels).
+    idx = image[:, :, -1:].array
+
+    # All values must be 0, 1, or 2.
+    assert dr.all((idx == 0.0) | (idx == 1.0) | (idx == 2.0)), \
+        "shape_index contains values outside {0, 1, 2} -- likely raw pointer reinterpret"
+
+    # Both shapes must appear in the output.
+    assert dr.any(idx == 1.0), "shape index 1 not found in AOV output"
+    assert dr.any(idx == 2.0), "shape index 2 not found in AOV output"