diff --git a/src/cosmology.c b/src/cosmology.c
index c4523b982aba5e564163d1033a359d991a87c717..b696d1f8740823d5a808c721c2f756198507279d 100644
--- a/src/cosmology.c
+++ b/src/cosmology.c
@@ -191,7 +191,7 @@ void cosmology_update(struct cosmology *c, const struct phys_const *phys_const,
       3. * c->H * c->H / (8. * M_PI * phys_const->const_newton_G);
 
   /* Mean density */
-  c->mean_density = c->critical_density_0 * a3_inv;
+  c->mean_density = c->critical_density_0 * c->a3_inv;
 
   /* Time-step conversion factor */
   c->time_step_factor = c->H;