diff --git a/Tests/Data/RichardsMechanics/mechanics_linear.prj b/Tests/Data/RichardsMechanics/mechanics_linear.prj
index 6d0e8ae8c35a2d9c296bea6de3e29dafe4da30c5..fe83ced3a89dcbd670552d3bb3881ff82f12227f 100644
--- a/Tests/Data/RichardsMechanics/mechanics_linear.prj
+++ b/Tests/Data/RichardsMechanics/mechanics_linear.prj
@@ -21,7 +21,7 @@
                 <relative_epsilons>1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8 1e-8</relative_epsilons>
             </jacobian_assembler>
             -->
-            <constitutive_relation>
+            <constitutive_relation id="0">
                 <type>LinearElasticIsotropic</type>
                 <youngs_modulus>E</youngs_modulus>
                 <poissons_ratio>nu</poissons_ratio>