Join throws error: Cannot read properties of null (reading 'getCount')

[olivierchatry]

Describe the bug

Using join on a particular GLTF, after a merge, crash

To Reproduce
call join on a mesh
I'm wondering if does not relate to an issue weld had and was fixed about gltf with points.

Expected behavior
Should not crash.

Versions:

• Version: 4.2.1
• Environment: CLI, Ubunutu, WSL2

Additional context
I'm not sure I can provide the GLTF file as it is not public data.

[donmccurdy]

Hi @olivierchatry! I'll venture to guess that the error is thrown at this line in the join() function:

glTF-Transform/packages/functions/src/join.ts

Lines 248 to 250 in 3816a84

	const dstPrim = joinPrimitives(prims);
	const dstVertexCount = dstPrim.listAttributes()[0].getCount();
	dstMesh.addPrimitive(dstPrim);

Reaching this step in processing and — presumably — finding no vertex attributes in the joined primitive is unexpected, and I'm not sure what inputs would cause it. Possibly some of the input mesh primitives don't have any vertex attributes, or all vertex attributes are 0-length?

I don't think I'll be able to investigate successfully without example data, but if you're able to share a sample of the data, or to debug further yourself, that would be very welcome!

[olivierchatry]

I can definitely try to debug ! Will try on Monday.

…

On Fri, Oct 31, 2025, 20:42 Don McCurdy ***@***.***> wrote: *donmccurdy* left a comment (donmccurdy/glTF-Transform#1748) <#1748 (comment)> Hi @olivierchatry <https://github.com/olivierchatry>! I'll venture to guess that the error is thrown at this line in the join() function: https://github.com/donmccurdy/glTF-Transform/blob/3816a842b191c66496c08972cf9fdc79196d24b0/packages/functions/src/join.ts#L248-L250 Reaching this step in processing and — presumably — finding no vertex attributes in the joined primitive is unexpected, and I'm not sure what inputs would cause it. Possibly some of the input mesh primitives don't have any vertex attributes, or all vertex attributes are 0-length? I don't think I'll be able to investigate successfully without example data, but if you're able to share a sample of the data, or to debug further yourself, that would be very welcome! — Reply to this email directly, view it on GitHub <#1748 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAJVOG7LKIVLHDJVYFYKJLD32O3QTAVCNFSM6AAAAACKYJ7GBGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTINZUGY2TMOJQGQ> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[olivierchatry]

So debugged it, and the crash happen when it tries to get the vertex count of a primitive :

export function getPrimitiveVertexCount(prim: Primitive, method: VertexCountMethod): number {
	const position = prim.getAttribute('POSITION')!;
	const indices = prim.getIndices();

	switch (method) {
		case VertexCountMethod.RENDER:
			return indices ? indices.getCount() : position.getCount();
		case VertexCountMethod.RENDER_CACHED:
			return indices ? new Set(indices.getArray()).size : position.getCount();
		case VertexCountMethod.UPLOAD_NAIVE:
		case VertexCountMethod.UPLOAD:
			return position.getCount();
		case VertexCountMethod.DISTINCT:
		case VertexCountMethod.DISTINCT_POSITION:
			return _assertNotImplemented(method);
		case VertexCountMethod.UNUSED:
			return indices ? position.getCount() - new Set(indices.getArray()).size : 0;
		default:
			return _assertUnreachable(method);
	}
}

What I did was change the prune function to delete null position primitive when cleaning up the mesh.

		// Prune empty Meshes.
		if (propertyTypes.has(PropertyType.MESH)) {
			for (const mesh of root.listMeshes()) {
				// Make sure that all primitive have a position
				for (const prim of mesh.listPrimitives()) {
					if (!prim.getAttribute('POSITION')) {
						prim.dispose()
					}
				}
				if (mesh.listPrimitives().length > 0) continue
				mesh.dispose();
			}
		}

It works, but the model is borked anyway ( it's a convertion from a navis to a step file, them to a gltf ). So I'm not sure if my code is of any interest for you ( and it does not fix the crash, it just mitigate it, but you need to call prune before ).

[donmccurdy]

Thanks @olivierchatry! I think it sounds reasonable that getPrimitiveVertexCount should just return 0 given a primitive with no vertex attributes, so I'll keep this issue open for that purpose. There might be other APIs within the library where handling zero-attribute primitives doesn't make as much sense, so I can't guarantee all such cases would be worked around, but I think this one is likely worth the change.