diff --git a/frontend/src/lib/document/transcription/useTranscriptionBlocks.svelte.test.ts b/frontend/src/lib/document/transcription/useTranscriptionBlocks.svelte.test.ts
index dd837f48..044aaed3 100644
--- a/frontend/src/lib/document/transcription/useTranscriptionBlocks.svelte.test.ts
+++ b/frontend/src/lib/document/transcription/useTranscriptionBlocks.svelte.test.ts
@@ -280,6 +280,25 @@ describe('createTranscriptionBlocks.markAllReviewed', () => {
 		await expect(ctrl.markAllReviewed()).rejects.toThrow('INTERNAL_ERROR');
 		expect(ctrl.blocks[0].reviewed).toBe(false);
 	});
+
+	it('throws INTERNAL_ERROR when PUT returns non-JSON body (e.g. nginx 502)', async () => {
+		const fetchImpl = vi.fn(async (url: RequestInfo | URL, init?: RequestInit) => {
+			const u = url.toString();
+			const method = init?.method ?? 'GET';
+			if (u.includes('/review-all') && method === 'PUT') {
+				return new Response('Bad Gateway', { status: 502 });
+			}
+			return new Response(JSON.stringify([baseBlock({ id: 'b-1', reviewed: false })]), {
+				status: 200,
+				headers: { 'Content-Type': 'application/json' }
+			});
+		});
+
+		const ctrl = createTranscriptionBlocks({ documentId: () => 'doc-1', fetchImpl });
+		await ctrl.load();
+		await expect(ctrl.markAllReviewed()).rejects.toThrow('INTERNAL_ERROR');
+		expect(ctrl.blocks[0].reviewed).toBe(false);
+	});
 });
 
 describe('createTranscriptionBlocks.createFromDraw', () => {