diff --git a/tests/test_mechanics.py b/tests/test_mechanics.py
index 60baa44c..f27ed6a0 100644
--- a/tests/test_mechanics.py
+++ b/tests/test_mechanics.py
@@ -39,7 +39,7 @@ def test_simple():
     r = fem.RegionHexahedron(m, uniform=True)
     u = fem.FieldContainer([fem.Field(r, dim=3)])
 
-    b = fem.SolidBody(umat, u)
+    b = fem.SolidBody(umat, u, density=1.0)
     r = b.assemble.vector()
 
     K = b.assemble.matrix()