diff --git a/backend/src/templateFormatter.ts b/backend/src/templateFormatter.ts
index 0dcb7bde..af30fd4e 100644
--- a/backend/src/templateFormatter.ts
+++ b/backend/src/templateFormatter.ts
@@ -346,6 +346,7 @@ const baseValues = {
     return [...args].join("");
   },
   concatArr(arr, separator = "") {
+    if (!Array.isArray(arr)) return "";
     return arr.join(separator);
   },
   eq(...args) {