Index: png.c
===================================================================
--- png.c	(revision 29222)
+++ png.c	(revision 29223)
@@ -780,7 +780,7 @@
 
 	if (pcb_cam_end(&png_cam) == 0)
 		if (!png_cam.okempty)
-			pcb_message(PCB_MSG_ERROR, "png cam export for '%s' failed to produce any content\n", options[HA_cam].str);
+			pcb_message(PCB_MSG_ERROR, "png cam export for '%s' failed to produce any content (layer group missing)\n", options[HA_cam].str);
 }
 
 static int png_parse_arguments(pcb_hid_t *hid, int *argc, char ***argv)