diff --git a/misc/build.func b/misc/build.func
index 0b4f5d8..cd6ca74 100644
--- a/misc/build.func
+++ b/misc/build.func
@@ -781,7 +781,7 @@ config_file() {
       source "$CONFIG_FILE"
     fi
   fi
-  if [[ -n "$CT_ID" ]]; then
+  if [[ -n "${CT_ID}" ]]; then
     if [[ "$CT_ID" =~ ^([0-9]{3,4})-([0-9]{3,4})$ ]]; then
       MIN_ID=${BASH_REMATCH[1]}
       MAX_ID=${BASH_REMATCH[2]}