diff --git a/read-cache.c b/read-cache.c
index 3bf9ed4d43..edcb2c5d97 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2824,11 +2824,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 		}
 	}
 
-	if (!istate->version) {
+	if (!istate->version)
 		istate->version = get_index_format_default(the_repository);
-		if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0))
-			init_split_index(istate);
-	}
 
 	/* demote version 3 to version 2 when the latter suffices */
 	if (istate->version == 3 || istate->version == 2)
@@ -3255,7 +3252,7 @@ static int too_many_not_shared_entries(struct index_state *istate)
 int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		       unsigned flags)
 {
-	int new_shared_index, ret;
+	int new_shared_index, ret, test_split_index_env;
 	struct split_index *si = istate->split_index;
 
 	if (git_env_bool("GIT_TEST_CHECK_CACHE_TREE", 0))
@@ -3270,7 +3267,10 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 	if (istate->fsmonitor_last_update)
 		fill_fsmonitor_bitmap(istate);
 
-	if (!si || alternate_index_output ||
+	test_split_index_env = git_env_bool("GIT_TEST_SPLIT_INDEX", 0);
+
+	if ((!si && !test_split_index_env) ||
+	    alternate_index_output ||
 	    (istate->cache_changed & ~EXTMASK)) {
 		if (si)
 			oidclr(&si->base_oid);
@@ -3278,10 +3278,15 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		goto out;
 	}
 
-	if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0)) {
-		int v = si->base_oid.hash[0];
-		if ((v & 15) < 6)
+	if (test_split_index_env) {
+		if (!si) {
+			si = init_split_index(istate);
 			istate->cache_changed |= SPLIT_INDEX_ORDERED;
+		} else {
+			int v = si->base_oid.hash[0];
+			if ((v & 15) < 6)
+				istate->cache_changed |= SPLIT_INDEX_ORDERED;
+		}
 	}
 	if (too_many_not_shared_entries(istate))
 		istate->cache_changed |= SPLIT_INDEX_ORDERED;
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index e2aa0bd949..decd2527ed 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -533,4 +533,15 @@ test_expect_success 'reading split index at alternate location' '
 	test_cmp expect actual
 '
 
+test_expect_success 'GIT_TEST_SPLIT_INDEX works' '
+	git init git-test-split-index &&
+	(
+		cd git-test-split-index &&
+		>file &&
+		GIT_TEST_SPLIT_INDEX=1 git update-index --add file &&
+		ls -l .git/sharedindex.* >actual &&
+		test_line_count = 1 actual
+	)
+'
+
 test_done