diff --git a/tests/test_deformation_gradient.py b/tests/test_deformation_gradient.py
index 0163dfa..bef2b1b 100644
--- a/tests/test_deformation_gradient.py
+++ b/tests/test_deformation_gradient.py
@@ -113,7 +113,8 @@ def test_rescale_deformation_gradient(self, deformation_generator):
 
     def test_invariant1(self, deformation_generator):
         deformation_gradient = deformation_generator.uniaxial([2.0, 3.0])
-        assert (sum(np.diag(f)) == deformation_generator.invariant1(f) for f in deformation_gradient)
+        invariant1 = deformation_generator.invariant1(deformation_gradient)
+        assert (sum(np.diag(f)) == invariant1(f) for f in deformation_gradient)
 
     def test_invariant2(self, deformation_generator):
         deformation_gradient = deformation_generator.uniaxial([2.0, 3.0])