diff --git a/src/app/api/affiliates/offers/[id]/route.test.ts b/src/app/api/affiliates/offers/[id]/route.test.ts
index a98e359a..5f8b9aee 100644
--- a/src/app/api/affiliates/offers/[id]/route.test.ts
+++ b/src/app/api/affiliates/offers/[id]/route.test.ts
@@ -198,6 +198,28 @@ describe("PATCH /api/affiliates/offers/[id] - product_url validation (#137)", ()
     expect(body.error).toContain("string");
   });
 
+  it("rejects non-string title values before trimming", async () => {
+    const res = await PATCH(
+      makeRequest("offer-1", { title: { text: "New title" } }),
+      makeParams("offer-1")
+    );
+
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toBe("title must be a string");
+  });
+
+  it("rejects non-string description values before trimming", async () => {
+    const res = await PATCH(
+      makeRequest("offer-1", { description: ["New description"] }),
+      makeParams("offer-1")
+    );
+
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toBe("description must be a string");
+  });
+
   it("accepts valid https URL in product_url (#137)", async () => {
     // Mock the update chain to succeed
     mockFrom.mockReturnValue(chainable({ id: "offer-1", product_url: "https://example.com/product" }));
diff --git a/src/app/api/affiliates/offers/[id]/route.ts b/src/app/api/affiliates/offers/[id]/route.ts
index a53c3bd3..e2accf09 100644
--- a/src/app/api/affiliates/offers/[id]/route.ts
+++ b/src/app/api/affiliates/offers/[id]/route.ts
@@ -100,8 +100,18 @@ export async function PATCH(
     // Partial validation — only validate provided fields
     const updateData: Record<string, unknown> = { updated_at: new Date().toISOString() };
 
-    if (body.title !== undefined) updateData.title = body.title.trim();
-    if (body.description !== undefined) updateData.description = body.description.trim();
+    if (body.title !== undefined) {
+      if (typeof body.title !== "string") {
+        return NextResponse.json({ error: "title must be a string" }, { status: 400 });
+      }
+      updateData.title = body.title.trim();
+    }
+    if (body.description !== undefined) {
+      if (typeof body.description !== "string") {
+        return NextResponse.json({ error: "description must be a string" }, { status: 400 });
+      }
+      updateData.description = body.description.trim();
+    }
     if (body.product_url !== undefined) {
       if (body.product_url !== null && typeof body.product_url !== "string") {
         return NextResponse.json({ error: "product_url must be a string" }, { status: 400 });