diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index 7d20a74c5..9e9474e94 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -67,8 +67,9 @@ test_expect_success 'setup' '
 # "exec" commands are ran with the user shell by default, but this may
 # be non-POSIX. For example, if SHELL=zsh then ">file" doesn't work
 # to create a file. Unseting SHELL avoids such non-portable behavior
-# in tests.
+# in tests. It must be exported for it to take effect where needed.
 SHELL=
+export SHELL
 
 test_expect_success 'rebase -i with the exec command' '
 	git checkout master &&