diff --git a/src/display_buffer.cc b/src/display_buffer.cc
index ac2bb593..576e5a14 100644
--- a/src/display_buffer.cc
+++ b/src/display_buffer.cc
@@ -1,6 +1,7 @@
 #include "display_buffer.hh"
 
 #include "assert.hh"
+#include "utf8.hh"
 
 namespace Kakoune
 {
@@ -11,6 +12,8 @@ DisplayLine::iterator DisplayLine::split(iterator it, BufferIterator pos)
     assert(it->content.begin() < pos);
     assert(it->content.end() > pos);
 
+    assert(utf8::is_character_start(pos));
+
     DisplayAtom atom = *it;
     atom.content.m_end = pos;
     it->content.m_begin = pos;