diff --git a/frontend/src/routes/(app)/recipes/[id]/edit/+page.server.ts b/frontend/src/routes/(app)/recipes/[id]/edit/+page.server.ts
index d430fd3..b062dba 100644
--- a/frontend/src/routes/(app)/recipes/[id]/edit/+page.server.ts
+++ b/frontend/src/routes/(app)/recipes/[id]/edit/+page.server.ts
@@ -72,8 +72,8 @@ export const actions: Actions = {
 			params: { path: { id: params.id } },
 			body: {
 				name: name.trim(),
-				serves: serves ? Number(serves) || undefined : undefined,
-				cookTimeMin: cookTimeMin ? Number(cookTimeMin) || undefined : undefined,
+				serves: serves ? Number(serves) || null : null,
+				cookTimeMin: cookTimeMin ? Number(cookTimeMin) || null : null,
 				effort,
 				heroImageUrl,
 				ingredients: (parsedIngredients as { name: string; quantity: string; unit: string }[])
diff --git a/frontend/src/routes/(app)/recipes/[id]/edit/page.server.test.ts b/frontend/src/routes/(app)/recipes/[id]/edit/page.server.test.ts
index 9594326..5e7e1ef 100644
--- a/frontend/src/routes/(app)/recipes/[id]/edit/page.server.test.ts
+++ b/frontend/src/routes/(app)/recipes/[id]/edit/page.server.test.ts
@@ -178,6 +178,19 @@ describe('edit recipe page — update action', () => {
 		}));
 	});
 
+	it('sends null for serves and cookTimeMin when fields are blank', async () => {
+		mockPut.mockResolvedValue({ error: undefined });
+		const fd = makeFormData({ serves: '', cookTimeMin: '' });
+		await actions.update({
+			request: { formData: async () => fd },
+			fetch: vi.fn(),
+			params: { id: 'r1' }
+		} as any).catch(() => {});
+		const body = mockPut.mock.calls[0][1].body;
+		expect(body.serves).toBeNull();
+		expect(body.cookTimeMin).toBeNull();
+	});
+
 	it('sends heroImageUrl in PUT body when provided', async () => {
 		mockPut.mockResolvedValue({ error: undefined });
 		const fd = makeFormData({ heroImageUrl: 'data:image/jpeg;base64,abc123' });
diff --git a/frontend/src/routes/(app)/recipes/new/+page.server.ts b/frontend/src/routes/(app)/recipes/new/+page.server.ts
index 6f03c3e..c1f2e30 100644
--- a/frontend/src/routes/(app)/recipes/new/+page.server.ts
+++ b/frontend/src/routes/(app)/recipes/new/+page.server.ts
@@ -44,8 +44,8 @@ export const actions: Actions = {
 		const { error: apiError } = await api.POST('/v1/recipes', {
 			body: {
 				name: name.trim(),
-				serves: serves ? Number(serves) || undefined : undefined,
-				cookTimeMin: cookTimeMin ? Number(cookTimeMin) || undefined : undefined,
+				serves: serves ? Number(serves) || null : null,
+				cookTimeMin: cookTimeMin ? Number(cookTimeMin) || null : null,
 				effort,
 				heroImageUrl,
 				ingredients: (parsedIngredients as { name: string; quantity: string; unit: string }[])
diff --git a/frontend/src/routes/(app)/recipes/new/page.server.test.ts b/frontend/src/routes/(app)/recipes/new/page.server.test.ts
index 10625cc..a7fcc4d 100644
--- a/frontend/src/routes/(app)/recipes/new/page.server.test.ts
+++ b/frontend/src/routes/(app)/recipes/new/page.server.test.ts
@@ -145,6 +145,15 @@ describe('new recipe page — create action', () => {
 		expect(mockPost).toHaveBeenCalledWith('/v1/recipes', expect.objectContaining({ body: expect.objectContaining({ name: 'Test Rezept', effort: 'easy' }) }));
 	});
 
+	it('sends null for serves and cookTimeMin when fields are blank', async () => {
+		mockPost.mockResolvedValue({ error: undefined });
+		const fd = makeFormData({ serves: '', cookTimeMin: '' });
+		await actions.create({ request: { formData: async () => fd }, fetch: vi.fn() } as any).catch(() => {});
+		const body = mockPost.mock.calls[0][1].body;
+		expect(body.serves).toBeNull();
+		expect(body.cookTimeMin).toBeNull();
+	});
+
 	it('sends heroImageUrl in POST body when provided', async () => {
 		mockPost.mockResolvedValue({ error: undefined });
 		const fd = makeFormData({ heroImageUrl: 'data:image/jpeg;base64,abc123' });