diff --git a/data.py b/data.py
index 03290b8..1e34fe3 100644
--- a/data.py
+++ b/data.py
@@ -49,7 +49,7 @@ class Data(MutableMapping):
         if not os.path.exists(data_dir_path):
             os.mkdir(data_dir_path)
         self._cache = {}
-        self._hot_keys = []
+        self._hot_keys = set()
         self._trunning = False
         self._tevent = Event()
         self._thread = None
@@ -71,7 +71,7 @@ class Data(MutableMapping):
 
     def __setitem__(self, key, value):
         self._cache[key] = value
-        self._hot_keys.append(key)
+        self._hot_keys.add(key)
         self._schedule_save()
 
     def _schedule_save(self):
@@ -92,7 +92,7 @@ class Data(MutableMapping):
             to_write = self._cache[key]  # apparently thread-safe
             with open(data_path.format(key), "w+") as f:
                 f.write(json.dumps(to_write))
-        self._hot_keys = []
+        self._hot_keys = set()
         self._saving = False
 
     def __delitem__(self, key):
@@ -143,6 +143,8 @@ class Log:
             month = self._data[pdate.strftime("%Y-%m")]
             del month[pdate.strftime("%d")][-1]
             self._data[pdate.strftime("%Y-%m")] = month
+            if len(self._data[pdate.strftime("%Y-%m")][pdate.strftime("%d")]) == 0:
+                return None
             last = base64.b64decode(
                 self._data[pdate.strftime("%Y-%m")][pdate.strftime("%d")][-1].split()[1].encode("utf-8")).decode("utf-8")
         return last
@@ -160,11 +162,13 @@ class Report:
 
     def report(self):
         tmp = []
-        for e in self._data[self._date.strftime("%Y-%m")][self._date.strftime("%d")]:
-            tstr, b64str = e.split()
-            task = base64.b64decode(b64str.encode("utf-8")).decode("utf-8")
-            start_time = datetime.combine(self._date, datetime.strptime(tstr, "%H:%M").time())
-            tmp.append((task, start_time))
+        if self._date.strftime("%Y-%m") in self._data \
+                and self._date.strftime("%d") in self._data[self._date.strftime("%Y-%m")]:
+            for e in self._data[self._date.strftime("%Y-%m")][self._date.strftime("%d")]:
+                tstr, b64str = e.split()
+                task = base64.b64decode(b64str.encode("utf-8")).decode("utf-8")
+                start_time = datetime.combine(self._date, datetime.strptime(tstr, "%H:%M").time())
+                tmp.append((task, start_time))
         if self._date == date.today():
             tmp.append(("End", datetime.now()))
 
@@ -193,6 +197,8 @@ class Report:
             report = report[:-3]  # cut off sum display and end time
         else:
             report = report[:-2]  # cut off sum display
+        if not report:
+            return
         save_list = []
         for tstr, ttime, _ in report:
             b64str = base64.b64encode(tstr.encode("utf-8")).decode("utf-8")
@@ -206,6 +212,8 @@ class Report:
         self._data[self._date.strftime("%Y-%m")] = month
 
     def prev_next_avail(self):
+        if self._date.strftime("%Y-%m") not in self._data:
+            return False, False
         prev = (self._date - timedelta(days=1)).strftime("%d") in self._data[self._date.strftime("%Y-%m")]
         _next = (self._date + timedelta(days=1)).strftime("%d") in self._data[self._date.strftime("%Y-%m")]
         return prev, _next