diff --git a/src/buffer.cc b/src/buffer.cc
index 92089e83..d384f3bb 100644
--- a/src/buffer.cc
+++ b/src/buffer.cc
@@ -245,7 +245,8 @@ void Buffer::check_invariant() const
 
 void Buffer::do_insert(const BufferIterator& pos, const String& content)
 {
-    assert(pos.is_end() or utf8::is_character_start(pos));
+    assert(pos.is_valid() and (pos.is_end() or utf8::is_character_start(pos)));
+    assert(not contains(content, '\0'));
     ++m_timestamp;
     ByteCount offset = pos.offset();
 
@@ -322,6 +323,8 @@ void Buffer::do_insert(const BufferIterator& pos, const String& content)
 
 void Buffer::do_erase(const BufferIterator& begin, const BufferIterator& end)
 {
+    assert(begin.is_valid());
+    assert(end.is_valid());
     assert(utf8::is_character_start(begin) and
            (end.is_end() or utf8::is_character_start(end)));
     ++m_timestamp;
diff --git a/src/dynamic_selection_list.cc b/src/dynamic_selection_list.cc
index d7eb5beb..90067505 100644
--- a/src/dynamic_selection_list.cc
+++ b/src/dynamic_selection_list.cc
@@ -64,7 +64,10 @@ DynamicSelectionList& DynamicSelectionList::operator=(SelectionList selections)
 void DynamicSelectionList::check_invariant() const
 {
     for (auto& sel : *this)
+    {
         assert(m_buffer == &sel.buffer());
+        sel.check_invariant();
+    }
 }
 
 void DynamicSelectionList::on_insert(const BufferIterator& begin, const BufferIterator& end)
@@ -73,7 +76,6 @@ void DynamicSelectionList::on_insert(const BufferIterator& begin, const BufferIt
     {
         sel.first().on_insert(begin.coord(), end.coord());
         sel.last().on_insert(begin.coord(), end.coord());
-        sel.check_invariant();
     }
 }
 
@@ -83,7 +85,6 @@ void DynamicSelectionList::on_erase(const BufferIterator& begin, const BufferIte
     {
         sel.first().on_erase(begin.coord(), end.coord());
         sel.last().on_erase(begin.coord(), end.coord());
-        sel.check_invariant();
     }
 }
 
diff --git a/src/editor.cc b/src/editor.cc
index 9fcc0f15..4bb9f464 100644
--- a/src/editor.cc
+++ b/src/editor.cc
@@ -334,12 +334,15 @@ public:
 
     void on_insert(const BufferIterator& begin, const BufferIterator& end)
     {
+        assert(begin.is_valid());
+        assert(end.is_valid());
         m_first = begin;
         m_last = utf8::previous(end);
     }
 
     void on_erase(const BufferIterator& begin, const BufferIterator& end)
     {
+        assert(begin.is_valid());
         m_first = begin;
         if (m_first >= m_buffer.end())
             m_first = utf8::previous(m_buffer.end());
diff --git a/src/selection.cc b/src/selection.cc
index bc156f26..77511c00 100644
--- a/src/selection.cc
+++ b/src/selection.cc
@@ -26,8 +26,10 @@ BufferIterator Range::end() const
 
 void Range::check_invariant() const
 {
-    assert(utf8::is_character_start(first()));
-    assert(utf8::is_character_start(last()));
+    assert(m_first.is_valid());
+    assert(m_last.is_valid());
+    assert(utf8::is_character_start(m_first));
+    assert(utf8::is_character_start(m_last));
 }
 
 static void avoid_eol(BufferIterator& it)