diff --git a/src/buffer.cc b/src/buffer.cc
index 12597b69..d74af40c 100644
--- a/src/buffer.cc
+++ b/src/buffer.cc
@@ -238,6 +238,7 @@ void Buffer::check_invariant() const
         assert(line.start == start);
         assert(line.length() > 0);
         assert(line.content.back() == '\n');
+        assert(find(line.content, '\n') == line.content.end()-1);
         start += line.length();
     }
 }