diff --git a/apps/web/lib/sponsor-amount.test.ts b/apps/web/lib/sponsor-amount.test.ts
index 7866f78..5a3f9a1 100644
--- a/apps/web/lib/sponsor-amount.test.ts
+++ b/apps/web/lib/sponsor-amount.test.ts
@@ -34,4 +34,19 @@ describe("normalizeSponsorAmountUsd", () => {
       error: "Enter a valid sponsorship amount.",
     });
   });
+
+  it("rejects non-string and non-number values before numeric coercion", () => {
+    expect(normalizeSponsorAmountUsd([5])).toEqual({
+      ok: false,
+      error: "Enter a valid sponsorship amount.",
+    });
+    expect(normalizeSponsorAmountUsd({ valueOf: () => 5 })).toEqual({
+      ok: false,
+      error: "Enter a valid sponsorship amount.",
+    });
+    expect(normalizeSponsorAmountUsd(true)).toEqual({
+      ok: false,
+      error: "Enter a valid sponsorship amount.",
+    });
+  });
 });
diff --git a/apps/web/lib/sponsor-amount.ts b/apps/web/lib/sponsor-amount.ts
index 72e6c4a..45f1a83 100644
--- a/apps/web/lib/sponsor-amount.ts
+++ b/apps/web/lib/sponsor-amount.ts
@@ -6,6 +6,9 @@ export function normalizeSponsorAmountUsd(
   if (typeof value === "string" && !/^\d+(?:\.\d+)?$/.test(value.trim())) {
     return { ok: false, error: "Enter a valid sponsorship amount." };
   }
+  if (typeof value !== "string" && typeof value !== "number") {
+    return { ok: false, error: "Enter a valid sponsorship amount." };
+  }
 
   const amount = Math.round(Number(value) * 100) / 100;
   if (!Number.isFinite(amount)) {