diff --git a/tests/test_mapping.py b/tests/test_mapping.py
index 6b98d3f0..7c1375f5 100644
--- a/tests/test_mapping.py
+++ b/tests/test_mapping.py
@@ -100,4 +100,4 @@ def test_mapping_memory_optimization():
     })
     orig = MappingAffine(m)
     opt = MappingAffine(m, tind=m.subdomains['omega0'])
-    assert orig.detA > opt.detA
+    assert len(orig.detA) > len(opt.detA)