diff --git a/frontend/src/ts/test/test-input.ts b/frontend/src/ts/test/test-input.ts
index 27910a80cdcc..b62359165529 100644
--- a/frontend/src/ts/test/test-input.ts
+++ b/frontend/src/ts/test/test-input.ts
@@ -212,7 +212,11 @@ export const corrected = new Corrected();
 export let keypressCountHistory: number[] = [];
 let currentKeypressCount = 0;
 export let currentBurstStart = 0;
-export let missedWords: Record<string, number> = {};
+type MissedWordsType = Record<string, number>;
+// We're using Object.create(null) to make sure that __proto__ won't have any special meaning when it's used to index the missedWords object (so if a user mistypes the word __proto__ it will appear in the practise words test)
+export let missedWords: MissedWordsType = Object.create(
+  null,
+) as MissedWordsType;
 export let accuracy = {
   correct: 0,
   incorrect: 0,
@@ -542,7 +546,7 @@ export function restart(): void {
     words: [],
   };
   currentBurstStart = 0;
-  missedWords = {};
+  missedWords = Object.create(null) as MissedWordsType;
   accuracy = {
     correct: 0,
     incorrect: 0,