diff --git a/ext/js/general/cache-map.js b/ext/js/general/cache-map.js
index 8650d8e694..a995b8c7b6 100644
--- a/ext/js/general/cache-map.js
+++ b/ext/js/general/cache-map.js
@@ -29,6 +29,7 @@ export class CacheMap {
      */
     constructor(maxSize) {
         if (!(
+            typeof maxSize === 'number' &&
             Number.isFinite(maxSize) &&
             maxSize >= 0 &&
             Math.floor(maxSize) === maxSize
diff --git a/test/cache-map.test.js b/test/cache-map.test.js
index 9db306772b..52ada57ca8 100644
--- a/test/cache-map.test.js
+++ b/test/cache-map.test.js
@@ -31,7 +31,9 @@ function testConstructor() {
             [true,  () => new CacheMap(-1)],
             [true,  () => new CacheMap(1.5)],
             [true,  () => new CacheMap(Number.NaN)],
-            [true,  () => new CacheMap(Number.POSITIVE_INFINITY)]
+            [true,  () => new CacheMap(Number.POSITIVE_INFINITY)],
+            // @ts-expect-error - Ignore because it should throw an error
+            [true,  () => new CacheMap('a')]
         ];
 
         for (const [throws, create] of data) {