diff --git a/src/utils.js b/src/utils.js
index 523600a..8f68561 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -1059,7 +1059,7 @@ function expandObjectIds(input) {
       for (const [key, val] of Object.entries(value)) {
         if (key === '_id') {
           // Do not expand keys that are already named _id
-          result[key] = val;
+          result[key] = val.toString();
         } else if (isObjectId(val)) {
           result[key] = { _id: val };
         } else if (Array.isArray(val)) {