diff --git a/diff-lib.c b/diff-lib.c
index 082e249fc3c..b73cc1859a4 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -606,10 +606,12 @@ int do_diff_cache(const struct object_id *tree_oid, struct diff_options *opt)
 
 	repo_init_revisions(opt->repo, &revs, NULL);
 	copy_pathspec(&revs.prune_data, &opt->pathspec);
+	diff_setup_done(&revs.diffopt);
 	revs.diffopt = *opt;
 
 	if (diff_cache(&revs, tree_oid, NULL, 1))
 		exit(128);
+	clear_pathspec(&revs.prune_data);
 	return 0;
 }