diff --git a/python/demo/demo_mixed-topology.py b/python/demo/demo_mixed-topology.py
index fe040fd027..579ffebba9 100644
--- a/python/demo/demo_mixed-topology.py
+++ b/python/demo/demo_mixed-topology.py
@@ -104,6 +104,8 @@
     a += [(ufl.inner(ufl.grad(u), ufl.grad(v)) - k**2 * u * v) * ufl.dx]
 
 # Compile the form
+# FIXME: For the time being, since UFL doesn't understand mixed topology meshes,
+# we have to call mixed_topology_form instead of form.
 a_form = mixed_topology_form(a)
 
 # Assemble the matrix