diff --git a/src/pi/main.py b/src/pi/main.py
index 06892c2..bb4d983 100644
--- a/src/pi/main.py
+++ b/src/pi/main.py
@@ -240,14 +240,14 @@
         target: 更新対象の dataclass インスタンス
         updates: フィールド名と値の辞書
     """
-    fields = {
-        f.name: f.type
-        for f in dataclasses.fields(target)
+    field_names = {
+        f.name for f in dataclasses.fields(target)
     }
     for key, value in updates.items():
-        if key not in fields:
+        if key not in field_names:
             continue
-        expected = fields[key]
+        current = getattr(target, key)
+        expected = type(current)
         # int フィールドに float が来た場合は変換を許容
         if expected is int and isinstance(value, float):
             value = int(value)