diff --git a/dir.c b/dir.c
index 1339b1a4b2477d..2a801fcae905ed 100644
--- a/dir.c
+++ b/dir.c
@@ -1420,7 +1420,7 @@ enum pattern_match_result path_matches_pattern_list(
 	struct path_pattern *pattern;
 	struct strbuf parent_pathname = STRBUF_INIT;
 	int result = NOT_MATCHED;
-	const char *slash_pos;
+	size_t slash_pos;
 
 	/*
 	 * The virtual file system data is used to prevent git from traversing
@@ -1461,10 +1461,11 @@ enum pattern_match_result path_matches_pattern_list(
 	 */
 	if (parent_pathname.len > 0 &&
 	    parent_pathname.buf[parent_pathname.len - 1] == '/') {
-		slash_pos = parent_pathname.buf + parent_pathname.len - 1;
+		slash_pos = parent_pathname.len - 1;
 		strbuf_add(&parent_pathname, "-", 1);
 	} else {
-		slash_pos = strrchr(parent_pathname.buf, '/');
+		const char *slash_ptr = strrchr(parent_pathname.buf, '/');
+		slash_pos = slash_ptr ? slash_ptr - parent_pathname.buf : 0;
 	}
 
 	if (hashmap_contains_path(&pl->recursive_hashmap,
@@ -1473,13 +1474,13 @@ enum pattern_match_result path_matches_pattern_list(
 		goto done;
 	}
 
-	if (slash_pos == parent_pathname.buf) {
+	if (!slash_pos) {
 		/* include every file in root */
 		result = MATCHED;
 		goto done;
 	}
 
-	strbuf_setlen(&parent_pathname, slash_pos - parent_pathname.buf);
+	strbuf_setlen(&parent_pathname, slash_pos);
 
 	if (hashmap_contains_path(&pl->parent_hashmap, &parent_pathname)) {
 		result = MATCHED;
diff --git a/repo-settings.c b/repo-settings.c
index 237dbfe61d6fbf..586034739832f8 100644
--- a/repo-settings.c
+++ b/repo-settings.c
@@ -96,9 +96,9 @@ void prepare_repo_settings(struct repository *r)
 	r->settings.command_requires_full_index = 1;
 
 	/*
-	 * Initialize this as off.
+	 * Initialize this as on.
 	 */
-	r->settings.sparse_index = 0;
-	if (!repo_config_get_bool(r, "index.sparse", &value) && value)
-		r->settings.sparse_index = 1;
+	r->settings.sparse_index = 1;
+	if (!repo_config_get_bool(r, "index.sparse", &value) && !value)
+		r->settings.sparse_index = 0;
 }
diff --git a/sparse-index.c b/sparse-index.c
index 489306029dc36d..c56543b37a00b8 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -108,7 +108,7 @@ int set_sparse_index_config(struct repository *repo, int enable)
 	char *config_path = repo_git_path(repo, "config.worktree");
 	res = git_config_set_in_file_gently(config_path,
 					    "index.sparse",
-					    enable ? "true" : NULL);
+					    enable ? "true" : "false");
 	free(config_path);
 
 	prepare_repo_settings(repo);
@@ -130,7 +130,8 @@ static int index_has_unmerged_entries(struct index_state *istate)
 int convert_to_sparse(struct index_state *istate)
 {
 	int test_env;
-	if (istate->split_index || istate->sparse_index ||
+
+	if (istate->split_index || istate->sparse_index || !istate->cache_nr ||
 	    !core_apply_sparse_checkout || !core_sparse_checkout_cone)
 		return 0;
 
@@ -158,10 +159,16 @@ int convert_to_sparse(struct index_state *istate)
 			return 0;
 	}
 
-	if (!istate->sparse_checkout_patterns->use_cone_patterns) {
-		warning(_("attempting to use sparse-index without cone mode"));
-		return -1;
-	}
+	/*
+	 * We need cone-mode patterns to use sparse-index. If a user edits
+	 * their sparse-checkout file manually, then we can detect during
+	 * parsing that they are not actually using cone-mode patterns and
+	 * hence we need to abort this conversion _without error_. Warnings
+	 * already exist in the pattern parsing to inform the user of their
+	 * bad patterns.
+	 */
+	if (!istate->sparse_checkout_patterns->use_cone_patterns)
+		return 0;
 
 	/*
 	 * NEEDSWORK: If we have unmerged entries, then stay full.
@@ -170,10 +177,17 @@ int convert_to_sparse(struct index_state *istate)
 	if (index_has_unmerged_entries(istate))
 		return 0;
 
-	if (cache_tree_update(istate, 0)) {
-		warning(_("unable to update cache-tree, staying full"));
-		return -1;
-	}
+	/* Clear and recompute the cache-tree */
+	cache_tree_free(&istate->cache_tree);
+	/*
+	 * Silently return if there is a problem with the cache tree update,
+	 * which might just be due to a conflict state in some entry.
+	 *
+	 * This might create new tree objects, so be sure to use
+	 * WRITE_TREE_MISSING_OK.
+	 */
+	if (cache_tree_update(istate, WRITE_TREE_MISSING_OK))
+		return 0;
 
 	remove_fsmonitor(istate);
 
diff --git a/t/t1091-sparse-checkout-builtin.sh b/t/t1091-sparse-checkout-builtin.sh
index 38fc8340f5c9b7..92613f381c7d23 100755
--- a/t/t1091-sparse-checkout-builtin.sh
+++ b/t/t1091-sparse-checkout-builtin.sh
@@ -215,7 +215,7 @@ test_expect_success 'sparse-index enabled and disabled' '
 	test-tool -C repo read-cache --table >cache &&
 	! grep " tree " cache &&
 	git -C repo config --list >config &&
-	! grep index.sparse config
+	test_cmp_config -C repo false index.sparse
 '
 
 test_expect_success 'cone mode: init and set' '
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 0fb2a42749b0e6..b7dc5569b6e43f 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -118,6 +118,7 @@ init_repos () {
 	git -C sparse-index reset --hard &&
 
 	# initialize sparse-checkout definitions
+	git -C sparse-checkout config index.sparse false &&
 	git -C sparse-checkout sparse-checkout init --cone &&
 	git -C sparse-checkout sparse-checkout set deep &&
 	git -C sparse-index sparse-checkout init --cone --sparse-index &&
diff --git a/t/t7817-grep-sparse-checkout.sh b/t/t7817-grep-sparse-checkout.sh
index 590b99bbb6f7bb..547bae48619c12 100755
--- a/t/t7817-grep-sparse-checkout.sh
+++ b/t/t7817-grep-sparse-checkout.sh
@@ -49,7 +49,7 @@ test_expect_success 'setup' '
 		echo "text" >B/b &&
 		git add A B &&
 		git commit -m sub &&
-		git sparse-checkout init --cone &&
+		git sparse-checkout init --cone --no-sparse-index &&
 		git sparse-checkout set B
 	) &&