diff --git a/build-sbx.sh b/build-sbx.sh
index a461bec..f7e8258 100755
--- a/build-sbx.sh
+++ b/build-sbx.sh
@@ -15,6 +15,10 @@ set -o pipefail
 readonly KIT_PATH="$HOME/.sbx/kits/hypoport.ai"
 readonly TEMPLATE='127.0.0.1:62735/finmas.de/mux033/opencode-ops:latest'
 
+# This is intentionally global because the EXIT cleanup function runs after
+# `main` has returned and therefore cannot access `main`'s local variables.
+tmp=''
+
 require_command() {
     local command_name="$1"
     local message="$2"
@@ -79,6 +83,15 @@ prompt_component() {
     done
 }
 
+cleanup_temporary_directory() {
+    # EXIT traps run after `main` returns, so a variable declared local inside
+    # `main` is no longer available. `${tmp:-}` also keeps nounset from
+    # turning normal script completion into an error when no copy was made.
+    if [[ -n "${tmp:-}" ]]; then
+        rm -rf -- "$tmp"
+    fi
+}
+
 main() {
     # Check dependencies before asking questions. A missing tool should not
     # leave the user halfway through an interactive setup.
@@ -193,7 +206,7 @@ main() {
         done
     fi
 
-    local tmp='' api_key
+    local api_key
     if [[ "$build_from_scratch" == false ]]; then
         # `sbx cp` needs a host-side intermediary. `mktemp` gives concurrent
         # runs separate directories, and the EXIT trap removes the copy even
@@ -202,7 +215,7 @@ main() {
             printf 'Unable to create a temporary directory.\n' >&2
             exit 1
         }
-        trap 'rm -rf "${tmp:?}"' EXIT
+        trap cleanup_temporary_directory EXIT
 
         # Copy before creating the new sandbox so a failed creation cannot
         # destroy the only temporary copy of the source data.