diff --git a/src/domain/months.test.ts b/src/domain/months.test.ts
index 7b32bd2..c3da79c 100644
--- a/src/domain/months.test.ts
+++ b/src/domain/months.test.ts
@@ -66,3 +66,31 @@ test("monthsOfYear returns twelve padded months", () => {
 test("yearOf parses the year", () => {
   expect(yearOf("2026-09")).toBe(2026);
 });
+
+describe("month range validation", () => {
+  test("yearOf rejects out-of-range month 13", () => {
+    expect(() => yearOf("2026-13")).toThrow(/2026-13/);
+  });
+
+  test("yearOf rejects out-of-range month 00", () => {
+    expect(() => yearOf("2026-00")).toThrow(/2026-00/);
+  });
+
+  test("monthOf rejects out-of-range month 13 from IsoDate", () => {
+    expect(() => monthOf("2026-13-01")).toThrow(/2026-13-01/);
+  });
+
+  test("monthOf rejects out-of-range month 00 from IsoDate", () => {
+    expect(() => monthOf("2026-00-15")).toThrow(/2026-00-15/);
+  });
+
+  test("valid boundary month 01 works", () => {
+    expect(yearOf("2026-01")).toBe(2026);
+    expect(monthOf("2026-01-01")).toBe("2026-01");
+  });
+
+  test("valid boundary month 12 works", () => {
+    expect(yearOf("2026-12")).toBe(2026);
+    expect(monthOf("2026-12-31")).toBe("2026-12");
+  });
+});
diff --git a/src/domain/months.ts b/src/domain/months.ts
index d586cfe..6fb0e2c 100644
--- a/src/domain/months.ts
+++ b/src/domain/months.ts
@@ -3,7 +3,10 @@ import type { IsoDate, MonthId } from "./types.ts";
 function parse(id: MonthId): { year: number; month: number } {
   const match = /^(\d{4})-(\d{2})$/.exec(id);
   if (!match) throw new Error(`Invalid MonthId: ${id}`);
-  return { year: Number(match[1]), month: Number(match[2]) };
+  const year = Number(match[1]);
+  const month = Number(match[2]);
+  if (month < 1 || month > 12) throw new Error(`Invalid MonthId: ${id}`);
+  return { year, month };
 }
 
 function format(year: number, month: number): MonthId {
@@ -23,6 +26,8 @@ function fromOrdinal(ordinal: number): MonthId {
 export function monthOf(date: IsoDate): MonthId {
   const match = /^(\d{4})-(\d{2})/.exec(date);
   if (!match) throw new Error(`Invalid IsoDate: ${date}`);
+  const month = Number(match[2]);
+  if (month < 1 || month > 12) throw new Error(`Invalid IsoDate: ${date}`);
   return `${match[1]}-${match[2]}`;
 }