diff --git a/firedrake/assign.py b/firedrake/assign.py
index 8d5f4789d0..944c70ad4a 100644
--- a/firedrake/assign.py
+++ b/firedrake/assign.py
@@ -161,13 +161,15 @@ def __init__(self, assignee, expression, subset=None):
                 if coeff.ufl_element() != assignee.ufl_element():
                     raise ValueError("All functions in the expression must have the same "
                                      "element as the assignee")
-                source_meshes.add(extract_unique_domain(coeff))
+                source_meshes.add(extract_unique_domain(coeff, expand_mesh_sequence=False))
         if len(source_meshes) == 0:
             pass
         elif len(source_meshes) == 1:
-            target_mesh = extract_unique_domain(assignee)
+            target_mesh = extract_unique_domain(assignee, expand_mesh_sequence=False)
             source_mesh, = source_meshes
-            if target_mesh.submesh_youngest_common_ancester(source_mesh) is None:
+            if target_mesh is source_mesh:
+                pass
+            elif target_mesh.submesh_youngest_common_ancester(source_mesh) is None:
                 raise ValueError(
                     "All functions in the expression must be defined on a single domain "
                     "that is in the same submesh family as domain of the assignee"
diff --git a/tests/firedrake/regression/test_multiple_domains.py b/tests/firedrake/regression/test_multiple_domains.py
index 1a4443fc43..fb7c9bf04a 100644
--- a/tests/firedrake/regression/test_multiple_domains.py
+++ b/tests/firedrake/regression/test_multiple_domains.py
@@ -205,3 +205,17 @@ def test_multi_domain_assemble():
     a = inner(u[0], v[0])*dx(domain=mesh1) + inner(u[1], v[1])*dx(domain=mesh2)
     A = assemble(a)
     assert A.M.values.shape == (V.dim(), V.dim())
+
+
+def test_multidomain_assign_function(mesh1, mesh3):
+    V1 = FunctionSpace(mesh1, "DG", 0)
+    V2 = FunctionSpace(mesh3, "CG", 1)
+    Z = V1 * V2
+    z = Function(Z)
+    z.subfunctions[0].assign(42)
+    z.subfunctions[1].assign(67)
+
+    w = Function(Z)
+    w.assign(z)
+    for zsub, wsub in zip(z.subfunctions, w.subfunctions):
+        assert np.allclose(zsub.dat.data, wsub.dat.data)