(commit->date < revs->max_age))
                                continue;
                        if (add_parents_to_list(revs, commit, &revs->commits, NULL) < 0)
-                               return NULL;
+                               die("Failed to traverse parents of commit %s",
+                                   sha1_to_hex(commit->object.sha1));
                }
 
                switch (simplify_commit(revs, commit)) {
                case commit_ignore:
                        continue;
                case commit_error:
-                       return NULL;
+                       die("Failed to simplify parents of commit %s",
+                           sha1_to_hex(commit->object.sha1));
                default:
                        return commit;
                }