diff --git a/graphify/export.py b/graphify/export.py
index be44a9d03..217c1013e 100644
--- a/graphify/export.py
+++ b/graphify/export.py
@@ -912,14 +912,24 @@ def to_graphml(
         for k in [k for k in attrs if k.startswith("_")]:
             del attrs[k]
     # nx.write_graphml raises ValueError on None attribute values; replace with "".
+    # It also only accepts scalar attribute values, so dict/list-valued
+    # attributes (e.g. per-node "metadata", graph-level "hyperedges") must be
+    # serialized to a JSON string first, or the writer raises TypeError.
+    def _scrub(val):
+        if val is None:
+            return ""
+        if isinstance(val, (dict, list)):
+            return json.dumps(val)
+        return val
+
+    for key, val in list(H.graph.items()):
+        H.graph[key] = _scrub(val)
     for node_id in H.nodes():
         for key, val in list(H.nodes[node_id].items()):
-            if val is None:
-                H.nodes[node_id][key] = ""
+            H.nodes[node_id][key] = _scrub(val)
     for u, v in H.edges():
         for key, val in list(H.edges[u, v].items()):
-            if val is None:
-                H.edges[u, v][key] = ""
+            H.edges[u, v][key] = _scrub(val)
     nx.write_graphml(H, output_path)
 
 
diff --git a/tests/test_export.py b/tests/test_export.py
index 7194d8b78..b6ab84860 100644
--- a/tests/test_export.py
+++ b/tests/test_export.py
@@ -99,6 +99,25 @@ def test_to_graphml_tolerates_none_attribute_values():
         content = out.read_text()
         assert "<graphml" in content
 
+def test_to_graphml_tolerates_dict_and_list_attribute_values():
+    """nx.write_graphml only accepts scalar attribute values and raises
+    TypeError on a dict or list — e.g. a node's "metadata" dict, or a
+    graph-level "hyperedges" list (attach_hyperedges()). to_graphml must
+    JSON-serialize these instead of passing them through unmodified."""
+    G = make_graph()
+    communities = cluster(G)
+    a_node = next(iter(G.nodes()))
+    G.nodes[a_node]["metadata"] = {"kind": "file"}
+    if G.number_of_edges():
+        u, v = next(iter(G.edges()))
+        G.edges[u, v]["tags"] = ["a", "b"]
+    G.graph["hyperedges"] = [{"nodes": [a_node], "label": "x"}]
+    with tempfile.TemporaryDirectory() as tmp:
+        out = Path(tmp) / "graph.graphml"
+        to_graphml(G, communities, str(out))  # must not raise
+        content = out.read_text()
+        assert "<graphml" in content
+
 def test_to_html_creates_file():
     G = make_graph()
     communities = cluster(G)