diff --git a/landing/siwe.react.js b/landing/siwe.react.js
--- a/landing/siwe.react.js
+++ b/landing/siwe.react.js
@@ -21,6 +21,8 @@
 } from 'wagmi';
 import { publicProvider } from 'wagmi/providers/public';
 
+import type { SIWEWebViewMessage } from 'lib/types/siwe-types';
+
 import { SIWENonceContext } from './siwe-nonce-context.js';
 import css from './siwe.css';
 
@@ -57,6 +59,10 @@
   return message.prepareMessage();
 }
 
+function postMessageToNativeWebView(message: SIWEWebViewMessage) {
+  window.ReactNativeWebView?.postMessage?.(JSON.stringify(message));
+}
+
 async function signInWithEthereum(address: string, signer, nonce: string) {
   invariant(nonce, 'nonce must be present in signInWithEthereum');
   const message = createSiweMessage(
@@ -65,8 +71,12 @@
     nonce,
   );
   const signature = await signer.signMessage(message);
-  const messageToPost = JSON.stringify({ address, message, signature });
-  window.ReactNativeWebView?.postMessage?.(messageToPost);
+  postMessageToNativeWebView({
+    type: 'siwe_success',
+    address,
+    message,
+    signature,
+  });
   return signature;
 }
 
diff --git a/lib/types/siwe-types.js b/lib/types/siwe-types.js
--- a/lib/types/siwe-types.js
+++ b/lib/types/siwe-types.js
@@ -3,3 +3,13 @@
 export type SIWENonceResponse = {
   +nonce: string,
 };
+
+// This is a message that the rendered webpage (landing/siwe.react.js) uses to
+// communicate back to the React Native WebView that is rendering it
+// (native/account/siwe-panel.react.js)
+export type SIWEWebViewMessage = {
+  +type: 'siwe_success',
+  +address: string,
+  +message: string,
+  +signature: string,
+};
diff --git a/native/account/siwe-panel.react.js b/native/account/siwe-panel.react.js
--- a/native/account/siwe-panel.react.js
+++ b/native/account/siwe-panel.react.js
@@ -12,6 +12,7 @@
 import { registerActionTypes, register } from 'lib/actions/user-actions';
 import { createLoadingStatusSelector } from 'lib/selectors/loading-selectors';
 import type { LogInStartingPayload } from 'lib/types/account-types';
+import type { SIWEWebViewMessage } from 'lib/types/siwe-types';
 import {
   useServerCall,
   useDispatchActionPromise,
@@ -83,12 +84,12 @@
   );
   const handleMessage = React.useCallback(
     event => {
-      const {
-        nativeEvent: { data },
-      } = event;
-      const { address, signature } = JSON.parse(data);
-      if (address && signature) {
-        handleSIWE({ address, signature });
+      const data: SIWEWebViewMessage = JSON.parse(event.nativeEvent.data);
+      if (data.type === 'siwe_success') {
+        const { address, signature } = data;
+        if (address && signature) {
+          handleSIWE({ address, signature });
+        }
       }
     },
     [handleSIWE],