diff --git a/Server/Database/Database.cpp b/Server/Database/Database.cpp
index 492fa25..fd93b63 100644
--- a/Server/Database/Database.cpp
+++ b/Server/Database/Database.cpp
@@ -47,9 +47,13 @@ Tasks Database::tasks() {
     std::unordered_map<int, Task *> tasks;
     for (auto iterator = ret.begin(); iterator != ret.end();) {
         if (iterator->parentId >= 0) {
-            auto parentTask = tasks.at(iterator->parentId);
-            parentTask->children.push_back(*iterator);
-            tasks.insert({iterator->id, &parentTask->children.back()});
+            if (tasks.count(iterator->parentId) > 0) {
+                auto parentTask = tasks.at(iterator->parentId);
+                parentTask->children.push_back(*iterator);
+                tasks.insert({iterator->id, &parentTask->children.back()});
+            } else {
+                LOG(warning) << "task`s parent id " << iterator->parentId << " not existed.";
+            }
             iterator = ret.erase(iterator);
         } else {
             tasks.insert({iterator->id, &(*iterator)});