diff --git a/map/src/map.hpp b/map/src/map.hpp
index 9d8b727..32a4a52 100644
--- a/map/src/map.hpp
+++ b/map/src/map.hpp
@@ -432,6 +432,7 @@ class map {
     iterator &operator--() {
       if (raw_pointer == nullptr) {
         if (domain == nullptr) throw invalid_iterator();
+        if (domain->tree_root == nullptr) throw invalid_iterator();
         raw_pointer = domain->tree_root;
         while (raw_pointer->right != nullptr) raw_pointer = raw_pointer->right;
         return *this;
@@ -464,13 +465,13 @@ class map {
       if (raw_pointer == nullptr) throw invalid_iterator();
       return raw_pointer->val;
     }
-    bool operator==(const iterator &rhs) const { return raw_pointer == rhs.raw_pointer; }
-    bool operator==(const const_iterator &rhs) const { return raw_pointer == rhs.raw_pointer; }
+    bool operator==(const iterator &rhs) const { return domain == rhs.domain && raw_pointer == rhs.raw_pointer; }
+    bool operator==(const const_iterator &rhs) const { return domain == rhs.domain && raw_pointer == rhs.raw_pointer; }
     /**
      * some other operator for iterator.
      */
-    bool operator!=(const iterator &rhs) const { return raw_pointer != rhs.raw_pointer; }
-    bool operator!=(const const_iterator &rhs) const { return raw_pointer != rhs.raw_pointer; }
+    bool operator!=(const iterator &rhs) const { return domain != rhs.domain || raw_pointer != rhs.raw_pointer; }
+    bool operator!=(const const_iterator &rhs) const { return domain != rhs.domain || raw_pointer != rhs.raw_pointer; }
 
     /**
      * for the support of it->first.
@@ -524,6 +525,7 @@ class map {
     const_iterator &operator--() {
       if (raw_pointer == nullptr) {
         if (domain == nullptr) throw invalid_iterator();
+        if (domain->tree_root == nullptr) throw invalid_iterator();
         raw_pointer = domain->tree_root;
         while (raw_pointer->right != nullptr) raw_pointer = raw_pointer->right;
         return *this;
@@ -556,13 +558,13 @@ class map {
       if (raw_pointer == nullptr) throw invalid_iterator();
       return raw_pointer->val;
     }
-    bool operator==(const iterator &rhs) const { return raw_pointer == rhs.raw_pointer; }
-    bool operator==(const const_iterator &rhs) const { return raw_pointer == rhs.raw_pointer; }
+    bool operator==(const iterator &rhs) const { return domain == rhs.domain && raw_pointer == rhs.raw_pointer; }
+    bool operator==(const const_iterator &rhs) const { return domain == rhs.domain && raw_pointer == rhs.raw_pointer; }
     /**
      * some other operator for iterator.
      */
-    bool operator!=(const iterator &rhs) const { return raw_pointer != rhs.raw_pointer; }
-    bool operator!=(const const_iterator &rhs) const { return raw_pointer != rhs.raw_pointer; }
+    bool operator!=(const iterator &rhs) const { return domain != rhs.domain || raw_pointer != rhs.raw_pointer; }
+    bool operator!=(const const_iterator &rhs) const { return domain != rhs.domain || raw_pointer != rhs.raw_pointer; }
     value_type *operator->() const noexcept { return &raw_pointer->val; }
   };
   map() : node_count(0), tree_root(nullptr) {}
@@ -607,11 +609,13 @@ class map {
    * If no such element exists, an exception of type `index_out_of_bound'
    */
   T &at(const Key &key) {
+    if (tree_root == nullptr) throw index_out_of_bound();
     RedBlackTreeNodeType *result = tree_root->Find(key);
     if (result == nullptr) throw index_out_of_bound();
     return result->val.second;
   }
   const T &at(const Key &key) const {
+    if (tree_root == nullptr) throw index_out_of_bound();
     RedBlackTreeNodeType *result = tree_root->Find(key);
     if (result == nullptr) throw index_out_of_bound();
     return result->val.second;