diff --git a/odc/loader/test_memreader.py b/odc/loader/test_memreader.py
index 0428eec..1eeddc6 100644
--- a/odc/loader/test_memreader.py
+++ b/odc/loader/test_memreader.py
@@ -173,11 +173,10 @@ def test_memreader_zarr(sample_ds: xr.Dataset):
     zarr = pytest.importorskip("zarr")
     assert zarr is not None
 
-    gbox = sample_ds.odc.geobox
+    _gbox = sample_ds.odc.geobox
     chunks = None
-
-    if isinstance(gbox, GCPGeoBox):
-        gbox = gbox.a
+    assert _gbox is not None
+    gbox = _gbox.approx if isinstance(_gbox, GCPGeoBox) else _gbox
 
     md_store: dict[str, bytes] = {}
     chunk_store: dict[str, bytes] = {}