diff --git a/experiments/benchmark_vs_urbevolve.py b/experiments/benchmark_vs_urbevolve.py
index f672049..7f6b008 100644
--- a/experiments/benchmark_vs_urbevolve.py
+++ b/experiments/benchmark_vs_urbevolve.py
@@ -50,7 +50,7 @@ def run_homemaker(seed: str, budget: int, cell: Path) -> dict:
     return {"series": series, "best_dom": out, "log_ok": proc.returncode == 0}
 
 
-def run_urbevolve(seed: str, budget: int, pop: int, cell: Path) -> dict:
+def run_urbevolve(seed: str, budget: int, cell: Path, pop: int) -> dict:
     shutil.copy(EX / "patterns.config", cell)
     shutil.copy(EX / seed, cell)
     proc = subprocess.run(
diff --git a/src/homemaker/operators.py b/src/homemaker/operators.py
index ad3f7f9..23f2796 100644
--- a/src/homemaker/operators.py
+++ b/src/homemaker/operators.py
@@ -101,7 +101,10 @@ def mutate_retype(root: dom.Node, rng: np.random.Generator,
 def mutate_swap(root: dom.Node, rng: np.random.Generator,
                 types: list[str]) -> tuple[dom.Node, str]:
     child = copy.deepcopy(root)
-    li, n = _pick(rng, _owned_branches(child))
+    cands = _owned_branches(child)
+    if not cands:  # undivided topology (e.g. a bare plot seed)
+        return _finalise(child), "swap noop"
+    li, n = _pick(rng, cands)
     n.left, n.right = n.right, n.left
     return _finalise(child), f"swap {li}/{n.id or 'root'}"
 
diff --git a/tests/test_operators.py b/tests/test_operators.py
index 77828df..73795b2 100644
--- a/tests/test_operators.py
+++ b/tests/test_operators.py
@@ -83,6 +83,18 @@ def test_relink_clears_stale_below_after_base_undivide():
     assert base.by_id(cands[0].id) is not None  # parent untouched
 
 
+def test_all_mutations_survive_undivided_tree():
+    # an undivided plot (init.dom-style seed) must never crash an operator
+    bare = dom.Node(type="O", node=[[0, 0], [10, 0], [10, 8], [0, 8]],
+                    height=2.7, wall_outer=0.25, wall_inner=0.08)
+    dom._link(bare)
+    for name, op in operators.MUTATIONS.items():
+        for seed in range(3):
+            child, desc = op(bare, np.random.default_rng(seed), TYPES)
+            assert desc, name
+            canonical(child)
+
+
 def test_crossover_yields_canonical_pair():
     a = genome.decode(genome.encode(dom.load(str(CORPUS / FILES[0]))))
     b = genome.decode(genome.encode(dom.load(str(CORPUS / FILES[1]))))