diff --git a/picker/code.gs b/picker/code.gs
index 3346d4214..3c0c363a4 100644
--- a/picker/code.gs
+++ b/picker/code.gs
@@ -36,11 +36,18 @@ function showPicker() {
     .setSandboxMode(HtmlService.SandboxMode.IFRAME);
   SpreadsheetApp.getUi().showModalDialog(html, "Select a file");
 }
+// Ensure the Drive API is enabled.
+if (!Drive) {
+  throw new Error('Please enable the Drive advanced service.');
+}
+
 /**
  * Checks that the file can be accessed.
+ * @param {string} fileId The ID of the file.
+ * @return {Object} The file resource.
  */
 function getFile(fileId) {
-  return Drive.Files.get(fileId, { fields: "*" });
+  return Drive.Files.get(fileId, {fields: '*'});
 }
 
 /**
diff --git a/picker/dialog.html b/picker/dialog.html
index 9d85af9eb..a694d015e 100644
--- a/picker/dialog.html
+++ b/picker/dialog.html
@@ -109,12 +109,27 @@
         }
       }
 
+      /**
+       * @typedef {Object} PickerResponse
+       * @property {string} action
+       * @property {PickerDocument[]} docs
+       */
+
+      /**
+       * @typedef {Object} PickerDocument
+       * @property {string} id
+       * @property {string} name
+       * @property {string} mimeType
+       * @property {string} url
+       * @property {string} lastEditedUtc
+       */
+
       /**
        * A callback function that extracts the chosen document's metadata from the
        * response object. For details on the response object, see
        * https://developers.google.com/picker/reference/picker.responseobject
        *
-       * @param {object} data The response object.
+       * @param {PickerResponse} data The response object.
        */
       function pickerCallback(data) {
         const action = data[google.picker.Response.ACTION];
@@ -128,7 +143,7 @@
       /**
        * Handles `"PICKED"` responsed from the Google Picker.
        *
-       * @param {object} data The response object.
+       * @param {PickerResponse} data The response object.
        */
       function handlePicked(data) {
         const doc = data[google.picker.Response.DOCUMENTS][0];