diff --git a/t/t0021-conversion.sh b/t/t0021-conversion.sh
index e10f5f787fca8b..c954c709ad2f50 100755
--- a/t/t0021-conversion.sh
+++ b/t/t0021-conversion.sh
@@ -390,6 +390,9 @@ test_expect_success PERL 'required process filter should filter data' '
 		EOF
 		test_cmp_exclude_clean expected.log debug.log &&
 
+		# Make sure that the file appears dirty, so checkout below has to
+		# run the configured filter.
+		touch test.r &&
 		filter_git checkout --quiet --no-progress empty-branch &&
 		cat >expected.log <<-EOF &&
 			START