diff --git a/examples/CosmologicalBox/cosmo.yml b/examples/CosmologicalBox/cosmo.yml
index 79fc02c1b41541c0f20ef24cda721dfa9db0ceba..a82cc5610b3317076a908902efdbd5b87d94262a 100644
--- a/examples/CosmologicalBox/cosmo.yml
+++ b/examples/CosmologicalBox/cosmo.yml
@@ -40,7 +40,6 @@ InitialConditions:
 
 Cosmology:
   Omega_m:        0.307
-  Omega_r:        0.000
   Omega_lambda:   0.693
   Omega_b:        0.0455
   h:              0.6777
diff --git a/src/cosmology.c b/src/cosmology.c
index 8955487c07c8743d05967a7de1da75147cdedd27..202e3e769db06e2de01b2f685cffee0f8986b28a 100644
--- a/src/cosmology.c
+++ b/src/cosmology.c
@@ -360,7 +360,7 @@ void cosmology_init(const struct swift_params *params,
 
   /* Read in the cosmological parameters */
   c->Omega_m = parser_get_param_double(params, "Cosmology:Omega_m");
-  c->Omega_r = parser_get_param_double(params, "Cosmology:Omega_r");
+  c->Omega_r = parser_get_opt_param_double(params, "Cosmology:Omega_r", 0.);
   c->Omega_lambda = parser_get_param_double(params, "Cosmology:Omega_lambda");
   c->Omega_b = parser_get_param_double(params, "Cosmology:Omega_b");
   c->w_0 = parser_get_opt_param_double(params, "Cosmology:w_0", -1.);