diff --git a/camlibs/ptp2/ptp.c b/camlibs/ptp2/ptp.c
index 486e60c72..7d0cf44b6 100644
--- a/camlibs/ptp2/ptp.c
+++ b/camlibs/ptp2/ptp.c
@@ -6346,7 +6346,14 @@ static int _cmp_ob (const void *a, const void *b)
 void
 ptp_objects_sort (PTPParams *params)
 {
+	int i;
+
 	qsort (params->objects, params->nrofobjects, sizeof(PTPObject), _cmp_ob);
+
+	for (i=0;i<params->nrofobjects-1;i++) {
+		if (params->objects[i].oid > params->objects[i+1].oid)
+			ptp_error(params, "wrong object sort order! %08x vs %08x at position %d\n", params->objects[i].oid, params->objects[i+1].oid, i);
+	}
 }
 
 /* Binary search in objects. Needs "objects" to be a sorted by objectid list!  */