diff --git a/etc/firebase-admin.api.md b/etc/firebase-admin.api.md
index e6b95c8871..b5b810a5e6 100644
--- a/etc/firebase-admin.api.md
+++ b/etc/firebase-admin.api.md
@@ -625,7 +625,7 @@ export namespace messaging {
     }
     export interface ApnsPayload {
         // (undocumented)
-        [customData: string]: object;
+        [customData: string]: any;
         aps: Aps;
     }
     export interface Aps {
diff --git a/src/messaging/index.ts b/src/messaging/index.ts
index 8e9657ea47..020ee7c875 100644
--- a/src/messaging/index.ts
+++ b/src/messaging/index.ts
@@ -299,7 +299,7 @@ export namespace messaging {
      * The `aps` dictionary to be included in the message.
      */
     aps: Aps;
-    [customData: string]: object;
+    [customData: string]: any;
   }
 
   /**