diff --git a/packages/editor/src/lib/glb-export.test.ts b/packages/editor/src/lib/glb-export.test.ts
index 11b4df7ee..10487802f 100644
--- a/packages/editor/src/lib/glb-export.test.ts
+++ b/packages/editor/src/lib/glb-export.test.ts
@@ -107,6 +107,26 @@ describe('prepareSceneForExport', () => {
     expect(visibleChildren).toHaveLength(1)
   })
 
+  test('fills undefined slots in an array material so no undefined survives the prune', () => {
+    // Multi-material / group geometry where one slot was never assigned:
+    // `mesh.material = [validMat, undefined]`. The scalar-null guard doesn't
+    // catch this (an array is never `== null`), so the undefined slot used to
+    // reach GLTFExporter and crash on `material.isShaderMaterial`.
+    const root = new THREE.Group()
+    const mesh = new THREE.Mesh(new THREE.BoxGeometry(1, 1, 1))
+    mesh.material = [nodeMaterial(), undefined as unknown as THREE.Material]
+    root.add(mesh)
+
+    const { scene } = prepareSceneForExport(root, {})
+
+    const exported = scene.children[0] as THREE.Mesh
+    const materials = exported.material as THREE.Material[]
+    expect(Array.isArray(materials)).toBe(true)
+    expect(materials).toHaveLength(2)
+    // No undefined/null slot survives; every slot is a real material.
+    expect(materials.every((m) => m != null)).toBe(true)
+  })
+
   test('stamps identity from the scene registry and strips other userData', () => {
     const root = new THREE.Group()
     const doorGroup = new THREE.Group()
diff --git a/packages/editor/src/lib/glb-export.ts b/packages/editor/src/lib/glb-export.ts
index b00931042..b1ad3977a 100644
--- a/packages/editor/src/lib/glb-export.ts
+++ b/packages/editor/src/lib/glb-export.ts
@@ -216,6 +216,20 @@ function pruneNonRenderableMeshes(root: THREE.Object3D, identityNodes: Set<THREE
       }
       return
     }
+    // The scalar branch above only catches `material == null`; an ARRAY material
+    // (multi-material / group geometry) is never `== null`, so a slot that was
+    // never assigned — e.g. `[validMat, undefined]` — slips through and reaches
+    // GLTFExporter, which reads `material.isShaderMaterial` on every group's
+    // material and crashes on the undefined slot. Don't drop the mesh (the other
+    // slots may be real geometry): fill any null/undefined holes with the hidden
+    // placeholder so the exporter never sees an undefined material.
+    if (
+      (renderable.isMesh === true || renderable.isLine === true || renderable.isPoints === true) &&
+      Array.isArray(renderable.material) &&
+      renderable.material.some((m) => m == null)
+    ) {
+      renderable.material = renderable.material.map((m) => m ?? PLACEHOLDER_MATERIAL)
+    }
     const mesh = object as THREE.Mesh
     if (!mesh.isMesh || isRenderableMesh(mesh)) return
     if (mesh.children.length > 0) {