diff --git a/src/mail/message_store.py b/src/mail/message_store.py
index 12be114..c9d3343 100644
--- a/src/mail/message_store.py
+++ b/src/mail/message_store.py
@@ -171,20 +171,25 @@ class MessageStore:
                 continue
 
             # Check if envelope matches query
-            subject = item.get("subject", "").lower()
+            # Use "or ''" to handle None values (key exists but value is None)
+            subject = (item.get("subject") or "").lower()
             from_info = item.get("from", {})
             from_name = (
-                from_info.get("name", "").lower() if isinstance(from_info, dict) else ""
+                (from_info.get("name") or "").lower()
+                if isinstance(from_info, dict)
+                else ""
             )
             from_addr = (
-                from_info.get("addr", "").lower() if isinstance(from_info, dict) else ""
+                (from_info.get("addr") or "").lower()
+                if isinstance(from_info, dict)
+                else ""
             )
             to_info = item.get("to", {})
             to_name = (
-                to_info.get("name", "").lower() if isinstance(to_info, dict) else ""
+                (to_info.get("name") or "").lower() if isinstance(to_info, dict) else ""
             )
             to_addr = (
-                to_info.get("addr", "").lower() if isinstance(to_info, dict) else ""
+                (to_info.get("addr") or "").lower() if isinstance(to_info, dict) else ""
             )
 
             if (