diff --git a/t/t8020-last-modified.sh b/t/t8020-last-modified.sh
index 75b18ee83b..a1ff351822 100755
--- a/t/t8020-last-modified.sh
+++ b/t/t8020-last-modified.sh
@@ -281,8 +281,10 @@ test_expect_success 'last-modified with Bloom filters and --show-trees' '
 		test_commit touch-b d/b &&
 
 		git commit-graph write --reachable --changed-paths &&
+		GIT_TEST_COMMIT_GRAPH=0 \
 		git -c core.commitGraph=false last-modified -t HEAD -- d/a \
 			>expect &&
+		GIT_TEST_COMMIT_GRAPH=1 \
 		git -c core.commitGraph=true last-modified -t HEAD -- d/a \
 			>actual &&