605c605,608
<                 rsvg_marker_render (startmarker, segments[i].p1x, segments[i].p1y, angle_from_vector (outgoing_vx, outgoing_vy), linewidth, ctx);
---
>                 if ( startmarker != NULL ) {
>                     rsvg_marker_render (startmarker, segments[i].p1x, segments[i].p1y,
>                             angle_from_vector (outgoing_vx, outgoing_vy), linewidth, ctx);
>                 }
635c638,639
<                 rsvg_marker_render (middlemarker, segments[i].p1x, segments[i].p1y, angle, linewidth, ctx);
---
>                 if ( middlemarker != NULL ) 
>                     rsvg_marker_render (middlemarker, segments[i].p1x, segments[i].p1y, angle, linewidth, ctx);
646c650,653
<             rsvg_marker_render (endmarker, segments[num_segments - 1].p4x, segments[num_segments - 1].p4y, angle_from_vector (incoming_vx, incoming_vy), linewidth, ctx);
---
>              if ( endmarker != NULL ) {
>                 rsvg_marker_render (endmarker, segments[num_segments - 1].p4x, segments[num_segments - 1].p4y,
>                         angle_from_vector (incoming_vx, incoming_vy), linewidth, ctx);
>             }
