diff --git a/src/pytest_reportlog/plugin.py b/src/pytest_reportlog/plugin.py
index d44d56f..0a58d4c 100644
--- a/src/pytest_reportlog/plugin.py
+++ b/src/pytest_reportlog/plugin.py
@@ -139,12 +139,23 @@ def pytest_terminal_summary(self, terminalreporter):
 
 
 def cleanup_unserializable(d: Dict[str, Any]) -> Dict[str, Any]:
-    """Return new dict with entries that are not json serializable by their str()."""
-    result = {}
-    for k, v in d.items():
+    """Return new dict with values converted to JSON-safe representations."""
+
+    def to_jsonable(value: Any) -> Any:
         try:
-            json.dumps({k: v})
-        except TypeError:
-            v = str(v)
-        result[k] = v
-    return result
+            json.dumps(value)
+        except (TypeError, ValueError):
+            pass
+        else:
+            return value
+
+        if isinstance(value, dict):
+            return {str(k): to_jsonable(v) for k, v in value.items()}
+        if isinstance(value, (list, tuple)):
+            return [to_jsonable(v) for v in value]
+        if isinstance(value, set):
+            return [to_jsonable(v) for v in sorted(value, key=str)]
+
+        return str(value)
+
+    return {str(k): to_jsonable(v) for k, v in d.items()}
diff --git a/tests/test_reportlog.py b/tests/test_reportlog.py
index 032df8b..8dec31a 100644
--- a/tests/test_reportlog.py
+++ b/tests/test_reportlog.py
@@ -182,8 +182,23 @@ def __str__(self):
     assert new == {"x": 1, "c": "C instance", "y": ["a", "b"]}
 
 
+def test_cleanup_unserializable_does_not_stringify_entire_user_properties():
+    """
+    Regression test for #12.
+
+    If a single user property value is not JSON-serializable (e.g. a set), we
+    should only sanitize that value instead of stringifying the entire
+    user_properties payload.
+    """
+    data = {"user_properties": [("hello", {"world", "mars"})]}
+    new = cleanup_unserializable(data)
+
+    assert new == {"user_properties": [["hello", ["mars", "world"]]]}
+
+
 def test_subtest(pytester, tmp_path):
     """Regression test for #90."""
+    pytest.importorskip("pytest_subtests")
     pytester.makepyfile("""
         def test_foo(subtests):
             with subtests.test():