diff --git a/frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts b/frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts
index 3a1eed5f..6067481d 100644
--- a/frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts
+++ b/frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts
@@ -7,9 +7,12 @@ import PdfViewer from './PdfViewer.svelte';
 afterEach(cleanup);
 
 function makeFakePdfjsLib() {
-	function TextLayerMock() {}
-	TextLayerMock.prototype.render = () => Promise.resolve();
-	TextLayerMock.prototype.cancel = () => {};
+	class TextLayerMock {
+		render() {
+			return Promise.resolve();
+		}
+		cancel() {}
+	}
 
 	return {
 		GlobalWorkerOptions: { workerSrc: '' },