diff --git a/diff-lib.c b/diff-lib.c
index 1cb97af22d..ae75eacbcc 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -446,7 +446,8 @@ int run_diff_index(struct rev_info *revs, int cached)
 	memset(&opts, 0, sizeof(opts));
 	opts.head_idx = 1;
 	opts.index_only = cached;
-	opts.diff_index_cached = cached;
+	opts.diff_index_cached = (cached &&
+				  !DIFF_OPT_TST(&revs->diffopt, FIND_COPIES_HARDER));
 	opts.merge = 1;
 	opts.fn = oneway_diff;
 	opts.unpack_data = revs;
@@ -503,7 +504,7 @@ int do_diff_cache(const unsigned char *tree_sha1, struct diff_options *opt)
 	memset(&opts, 0, sizeof(opts));
 	opts.head_idx = 1;
 	opts.index_only = 1;
-	opts.diff_index_cached = 1;
+	opts.diff_index_cached = !DIFF_OPT_TST(opt, FIND_COPIES_HARDER);
 	opts.merge = 1;
 	opts.fn = oneway_diff;
 	opts.unpack_data = &revs;
diff --git a/t/t4007-rename-3.sh b/t/t4007-rename-3.sh
index 25e7a83659..11502b7509 100755
--- a/t/t4007-rename-3.sh
+++ b/t/t4007-rename-3.sh
@@ -35,6 +35,11 @@ test_expect_success 'copy detection' '
 	compare_diff_raw current expected
 '
 
+test_expect_success 'copy detection, cached' '
+	git diff-index -C --find-copies-harder --cached $tree >current &&
+	compare_diff_raw current expected
+'
+
 # In the tree, there is only path0/COPYING.  In the cache, path0 and
 # path1 both have COPYING and the latter is a copy of path0/COPYING.
 # However when we say we care only about path1, we should just see