Index: src/obj_arc.c
===================================================================
--- src/obj_arc.c	(revision 27882)
+++ src/obj_arc.c	(revision 27883)
@@ -377,8 +377,16 @@
 /* copies an arc to buffer */
 void *pcb_arcop_add_to_buffer(pcb_opctx_t *ctx, pcb_layer_t *Layer, pcb_arc_t *Arc)
 {
-	pcb_layer_t *layer = &ctx->buffer.dst->Layer[pcb_layer_id(ctx->buffer.src, Layer)];
-	pcb_arc_t *a = pcb_arc_new(layer, Arc->X, Arc->Y,
+	pcb_arc_t *a;
+	pcb_layer_id_t lid = pcb_layer_id(ctx->buffer.src, Layer);
+	pcb_layer_t *layer;
+
+	/* the buffer may not have the specified layer, e.g. on loose subc subc-aux layer */
+	if (lid == -1)
+		return NULL;
+
+	layer = &ctx->buffer.dst->Layer[lid];
+	a = pcb_arc_new(layer, Arc->X, Arc->Y,
 		Arc->Width, Arc->Height, Arc->StartAngle, Arc->Delta,
 		Arc->Thickness, Arc->Clearance, pcb_flag_mask(Arc->Flags, PCB_FLAG_FOUND | ctx->buffer.extraflg), pcb_false);
 
Index: src/obj_line.c
===================================================================
--- src/obj_line.c	(revision 27882)
+++ src/obj_line.c	(revision 27883)
@@ -420,7 +420,14 @@
 void *pcb_lineop_add_to_buffer(pcb_opctx_t *ctx, pcb_layer_t *Layer, pcb_line_t *Line)
 {
 	pcb_line_t *line;
-	pcb_layer_t *layer = &ctx->buffer.dst->Layer[pcb_layer_id(ctx->buffer.src, Layer)];
+	pcb_layer_id_t lid = pcb_layer_id(ctx->buffer.src, Layer);
+	pcb_layer_t *layer;
+
+	/* the buffer may not have the specified layer, e.g. on loose subc subc-aux layer */
+	if (lid == -1)
+		return NULL;
+
+	layer = &ctx->buffer.dst->Layer[lid];
 	line = pcb_line_new(layer, Line->Point1.X, Line->Point1.Y,
 															Line->Point2.X, Line->Point2.Y,
 															Line->Thickness, Line->Clearance, pcb_flag_mask(Line->Flags, PCB_FLAG_FOUND | ctx->buffer.extraflg));
Index: src/obj_poly.c
===================================================================
--- src/obj_poly.c	(revision 27882)
+++ src/obj_poly.c	(revision 27883)
@@ -499,9 +499,16 @@
 /* copies a polygon to buffer */
 void *pcb_polyop_add_to_buffer(pcb_opctx_t *ctx, pcb_layer_t *Layer, pcb_poly_t *Polygon)
 {
-	pcb_layer_t *layer = &ctx->buffer.dst->Layer[pcb_layer_id(ctx->buffer.src, Layer)];
 	pcb_poly_t *polygon;
+	pcb_layer_id_t lid = pcb_layer_id(ctx->buffer.src, Layer);
+	pcb_layer_t *layer;
 
+	/* the buffer may not have the specified layer, e.g. on loose subc subc-aux layer */
+	if (lid == -1)
+		return NULL;
+
+	layer = &ctx->buffer.dst->Layer[lid];
+
 	polygon = pcb_poly_new(layer, Polygon->Clearance, Polygon->Flags);
 	pcb_poly_copy(polygon, Polygon, 0, 0);
 	pcb_poly_copy_meta(polygon, Polygon);