diff --git a/app/admin/utils.py b/app/admin/utils.py
index 7444dc4..c07b3a4 100644
--- a/app/admin/utils.py
+++ b/app/admin/utils.py
@@ -51,5 +51,21 @@ def log_admin_action(action, target_type=None, target_id=None, before=None, afte
 
 
 def model_to_dict(obj, fields):
-    """Return a plain dict of the named fields from an ORM object (for audit before/after)."""
-    return {f: getattr(obj, f, None) for f in fields}
+    """
+    Return a plain JSON-serialisable dict of the named fields from an ORM object.
+    Converts datetime -> ISO-8601 string, Decimal -> float, all others pass through.
+    Safe to use directly as AuditLog before_json / after_json values.
+    """
+    import datetime
+    from decimal import Decimal
+
+    result = {}
+    for f in fields:
+        val = getattr(obj, f, None)
+        if isinstance(val, (datetime.datetime, datetime.date)):
+            result[f] = val.isoformat()
+        elif isinstance(val, Decimal):
+            result[f] = float(val)
+        else:
+            result[f] = val
+    return result
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