diff --git a/include/__debug b/include/__debug
index 4e80c6f8d..e807fa5a4 100644
--- a/include/__debug
+++ b/include/__debug
@@ -148,6 +148,7 @@ public:
         ::new(__n) _C_node<_Cont>(__n->__c_, __n->__next_);
     }
 
+    void __insert_i(void* __i);
     __c_node* __insert_c(void* __c);
     void __erase_c(void* __c);
 
diff --git a/include/iterator b/include/iterator
index d5f1eb7da..05019fddc 100644
--- a/include/iterator
+++ b/include/iterator
@@ -1063,7 +1063,12 @@ public:
 private:
     iterator_type __i;
 public:
-    _LIBCPP_INLINE_VISIBILITY __wrap_iter() _NOEXCEPT {}
+    _LIBCPP_INLINE_VISIBILITY __wrap_iter() _NOEXCEPT
+    {
+#if _LIBCPP_DEBUG_LEVEL >= 2
+        __get_db()->__insert_i(this);
+#endif
+    }
     template <class _Up> _LIBCPP_INLINE_VISIBILITY __wrap_iter(const __wrap_iter<_Up>& __u,
         typename enable_if<is_convertible<_Up, iterator_type>::value>::type* = 0) _NOEXCEPT
         : __i(__u.base())
diff --git a/src/debug.cpp b/src/debug.cpp
index 04d506341..2d2d6432d 100644
--- a/src/debug.cpp
+++ b/src/debug.cpp
@@ -110,7 +110,9 @@ __libcpp_db::__find_c_from_i(void* __i) const
 {
     RLock _(mut());
     __i_node* i = __find_iterator(__i);
-    return i != nullptr ? (i->__c_ != nullptr ? i->__c_->__c_ : nullptr) : nullptr;
+    _LIBCPP_ASSERT(i != nullptr, "iterator constructed in translation unit with debug mode not enabled."
+                   "  #define _LIBCPP_DEBUG2 1 for that translation unit.");
+    return i->__c_ != nullptr ? i->__c_->__c_ : nullptr;
 }
 
 void
@@ -118,14 +120,20 @@ __libcpp_db::__insert_ic(void* __i, const void* __c)
 {
     WLock _(mut());
     __i_node* i = __insert_iterator(__i);
-    _LIBCPP_ASSERT(__cbeg_ != __cend_, "debug mode internal logic error __insert_ic A");
+    _LIBCPP_ASSERT(__cbeg_ != __cend_, "Container constructed in a translation unit with debug mode disabled."
+                   " But it is being used in a translation unit with debug mode enabled."
+                   " Enable it in the other translation unit with #define _LIBCPP_DEBUG2 1");
     size_t hc = hash<const void*>()(__c) % (__cend_ - __cbeg_);
     __c_node* c = __cbeg_[hc];
-    _LIBCPP_ASSERT(c != nullptr, "debug mode internal logic error __insert_ic B");
+    _LIBCPP_ASSERT(c != nullptr, "Container constructed in a translation unit with debug mode disabled."
+                   " But it is being used in a translation unit with debug mode enabled."
+                   " Enable it in the other translation unit with #define _LIBCPP_DEBUG2 1");
     while (c->__c_ != __c)
     {
         c = c->__next_;
-        _LIBCPP_ASSERT(c != nullptr, "debug mode internal logic error __insert_ic C");
+        _LIBCPP_ASSERT(c != nullptr, "Container constructed in a translation unit with debug mode disabled."
+                   " But it is being used in a translation unit with debug mode enabled."
+                   " Enable it in the other translation unit with #define _LIBCPP_DEBUG2 1");
     }
     c->__add(i);
     i->__c_ = c;
@@ -365,6 +373,13 @@ __libcpp_db::swap(void* c1, void* c2)
         (*p)->__c_ = p2;
 }
 
+void
+__libcpp_db::__insert_i(void* __i)
+{
+    WLock _(mut());
+    __insert_iterator(__i);
+}
+
 // private api
 
 _LIBCPP_HIDDEN