diff --git a/scripts/bench-batch-ubatch.sh b/scripts/bench-batch-ubatch.sh
index 47129da..084af65 100755
--- a/scripts/bench-batch-ubatch.sh
+++ b/scripts/bench-batch-ubatch.sh
@@ -74,7 +74,10 @@ service_start() {
 trap service_start EXIT
 
 run_cell() {
-    local b=$1 ub=$2 dir="$OUT/b${b}_ub${ub}" sampler start rc wall
+    # two statements: `local a=1 b=$a` expands b before the builtin assigns a, and
+    # set -u turns that into "b: unbound variable"
+    local b=$1 ub=$2
+    local dir="$OUT/b${b}_ub${ub}" sampler start rc wall
     mkdir -p "$dir"
     if [ -s "$dir/bench.json" ]; then
         echo "b=$b ub=$ub: bench.json present, skipping" >&2