diff --git a/src/webview/js/__tests__/rewriteHtml-test.js b/src/webview/js/__tests__/rewriteHtml-test.js
index 2f48d81e8e1..e446fe4034f 100644
--- a/src/webview/js/__tests__/rewriteHtml-test.js
+++ b/src/webview/js/__tests__/rewriteHtml-test.js
@@ -14,7 +14,7 @@ describe('rewriteHtml', () => {
     email: 'rosencrantz+guildenstern@zulip.com',
   };
 
-  const rewrite = (src: string): { input: string, before: string, after: string } => {
+  const rewrite = (src: string): {| input: string, before: string, after: string |} => {
     const img = document.createElement('img');
     img.setAttribute('src', src);
     const before = img.src;