Lessley Dennington [off-list ref] writes:

quoted

Hello everyone! See the following bug report pertaining to sparse-checkout
when run outside top-level directories.

In a bug report it is fine, but "outside top-level" usually means
above the top-level of the working tree.  Here, I think you meant
running in a subdirectory of the top-level.

Perhaps something along this line?

 builtin/sparse-checkout.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git c/builtin/sparse-checkout.c w/builtin/sparse-checkout.c
index 45e838d8f8..4e5efbb85e 100644
--- c/builtin/sparse-checkout.c
+++ w/builtin/sparse-checkout.c

@@ -753,6 +753,16 @@ static int sparse_checkout_set(int argc, const char **argv, const char *prefix)
        if (!core_sparse_checkout_cone && argc == 0) {
                argv = default_patterns;
                argc = default_patterns_nr;
+       } else if (argc && prefix && *prefix) {
+               /*
+                * The args are not pathspecs, so unfortunately we
+                * cannot imitate how cmd_add() uses parse_pathspec().
+                */
+               int i;
+               int prefix_len = strlen(prefix);
+
+               for (i = 0; i < argc; i++)
+                       argv[i] = prefix_path(prefix, prefix_len, argv[i]);
        }