diff --git a/packages/iframe-stamper/src/index.ts b/packages/iframe-stamper/src/index.ts
index fead60c9..943c2ca5 100644
--- a/packages/iframe-stamper/src/index.ts
+++ b/packages/iframe-stamper/src/index.ts
@@ -175,7 +175,13 @@ export class IframeStamper {
      * See https://developer.mozilla.org/en-US/docs/Web/API/MessagePort/postMessage#transfer
      */
     this.iframe.addEventListener("load", () => {
-      this.iframe.contentWindow?.postMessage(
+      if (!this.iframe.contentWindow || !this.iframe.contentWindow.postMessage) {
+        throw new Error(
+          "contentWindow or contentWindow.postMessage does not exist"
+        );
+      }
+
+      this.iframe.contentWindow.postMessage(
         { type: IframeEventType.TurnkeyInitMessageChannel },
         this.iframeOrigin,
         [this.messageChannel.port2]