diff --git a/t/t1600-index.sh b/t/t1600-index.sh
index 98c5a83db7..2f792bb8ff 100755
--- a/t/t1600-index.sh
+++ b/t/t1600-index.sh
@@ -68,6 +68,9 @@ test_expect_success 'out of bounds index.version issues warning' '
 test_expect_success 'index.skipHash config option' '
 	rm -f .git/index &&
 	git -c index.skipHash=true add a &&
+	test_trailing_hash .git/index >hash &&
+	echo $(test_oid zero) >expect &&
+	test_cmp expect hash &&
 	git fsck &&
 
 	test_commit start &&
@@ -76,6 +79,8 @@ test_expect_success 'index.skipHash config option' '
 	git -C sub config index.skipHash true &&
 	>sub/file &&
 	git -C sub add a &&
+	test_trailing_hash .git/modules/sub/index >hash &&
+	test_cmp expect hash &&
 	git -C sub fsck
 '
 
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 796093a7b3..60308843f8 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1875,3 +1875,11 @@ test_cmp_config_output () {
 	sort config-actual >sorted-actual &&
 	test_cmp sorted-expect sorted-actual
 }
+
+# Given a filename, extract its trailing hash as a hex string
+test_trailing_hash () {
+	local file="$1" &&
+	tail -c $(test_oid rawsz) "$file" |
+		test-tool hexdump |
+		sed "s/ //g"
+}