diff --git a/t/lib-commit-graph.sh b/t/lib-commit-graph.sh
index 4d3e7f0623..c8bd76a777 100755
--- a/t/lib-commit-graph.sh
+++ b/t/lib-commit-graph.sh
@@ -14,18 +14,27 @@ graph_git_two_modes() {
 	test_cmp expect output
 }
 
+# graph_git_behavior <name> <directory> <branch> <compare>
+#
+# Ensures that a handful of traversal operations produce the same
+# results with and without the commit-graph in use.
+#
+# NOTE: it is a bug to call this function with <directory> containing
+# any characters in $IFS.
 graph_git_behavior() {
 	MSG=$1
 	DIR=$2
 	BRANCH=$3
 	COMPARE=$4
 	test_expect_success "check normal git operations: $MSG" '
-		cd "$TRASH_DIRECTORY/$DIR" &&
-		graph_git_two_modes "log --oneline $BRANCH" &&
-		graph_git_two_modes "log --topo-order $BRANCH" &&
-		graph_git_two_modes "log --graph $COMPARE..$BRANCH" &&
-		graph_git_two_modes "branch -vv" &&
-		graph_git_two_modes "merge-base -a $BRANCH $COMPARE"
+		(
+			cd "$TRASH_DIRECTORY" &&
+			graph_git_two_modes "${DIR:+-C $DIR} log --oneline $BRANCH" &&
+			graph_git_two_modes "${DIR:+-C $DIR} log --topo-order $BRANCH" &&
+			graph_git_two_modes "${DIR:+-C $DIR} log --graph $COMPARE..$BRANCH" &&
+			graph_git_two_modes "${DIR:+-C $DIR} branch -vv" &&
+			graph_git_two_modes "${DIR:+-C $DIR} merge-base -a $BRANCH $COMPARE"
+		)
 	'
 }