diff --git a/firestore-send-email/functions/src/validation.ts b/firestore-send-email/functions/src/validation.ts
index 98f4e4ca7..cbc6441dd 100644
--- a/firestore-send-email/functions/src/validation.ts
+++ b/firestore-send-email/functions/src/validation.ts
@@ -53,7 +53,10 @@ export const attachmentSchema = z
   });
 
 export const attachmentsSchema = z
-  .array(attachmentSchema)
+  .array(attachmentSchema, {
+    invalid_type_error:
+      "Field 'attachments' must be an array. If you have a single attachment object, wrap it in an array (e.g., [{ filename: '...', path: '...' }])",
+  })
   .optional()
   .transform((attachments) =>
     attachments
@@ -184,12 +187,21 @@ function formatZodError(
     const path = issue.path.length > 0 ? issue.path.join(".") : context;
     switch (issue.code) {
       case "invalid_type":
+        if (issue.received === "undefined") {
+          const expected = issue.expected === "object" ? "map" : issue.expected;
+          const article = ["a", "e", "i", "o", "u"].includes(expected[0])
+            ? "an"
+            : "a";
+          return `Field '${path}' must be ${article} ${expected}`;
+        }
+
         if (issue.expected === "string") {
           return `Field '${path}' must be a string`;
         }
         if (issue.expected === "array") {
           return `Field '${path}' must be an array`;
         }
+
         if (issue.expected === "object") {
           return `Field '${path}' must be a map`;
         }