Christer Borang <mort(a)dtek.chalmers.se&gt; writes: The code in question is elt = point_in_line_start_cache (w, orig, (vtarget < 0 ? -vtarget : vtarget)); /* #### This assertion must be true before the if statements are hit but may possibly be wrong after the call to point_in_line_start_cache if orig is outside of the visible region of the buffer. Handle this. */ assert (elt >= 0); There is a line_start_cache related fix in 21.1.2., I suggest you upgrade. When you see it there, please report again. Jan