diff --git a/src/interpreter/lib/math.ts b/src/interpreter/lib/math.ts
index 023303c7..691753b0 100644
--- a/src/interpreter/lib/math.ts
+++ b/src/interpreter/lib/math.ts
@@ -218,7 +218,7 @@ export const stdMath: Record<`Math:${string}`, Value> = {
 		else if (options?.type !== undefined) {
 			throw new AiScriptRuntimeError('`options` must be an object if specified.');
 		}
-		if (seed.type !== 'num' && seed.type !== 'str' && seed.type !== 'null') throw new AiScriptRuntimeError('`seed` must be either number or string if specified.');
+		if (seed.type !== 'num' && seed.type !== 'str') throw new AiScriptRuntimeError('`seed` must be either number or string.');
 		switch (algo) {
 			case 'rc4_legacy':
 				return GenerateLegacyRandom(seed);
diff --git a/src/utils/random/chacha20.ts b/src/utils/random/chacha20.ts
index bea6df14..ab33ce18 100644
--- a/src/utils/random/chacha20.ts
+++ b/src/utils/random/chacha20.ts
@@ -53,20 +53,15 @@ export class ChaCha20 extends RandomBase {
 	private buffer: Uint8Array;
 	private filledBuffer: Uint8Array;
 	private counter: bigint;
-	constructor(seed?: Uint8Array | undefined) {
+	constructor(seed: Uint8Array) {
 		const keyNonceBytes = CHACHA_IVSIZE + CHACHA_KEYSIZE;
 		super();
-		let keynonce: Uint8Array;
-		if (typeof seed === 'undefined') {
-			keynonce = crypto.getRandomValues(new Uint8Array(keyNonceBytes));
-		} else {
-			keynonce = seed;
-			if (keynonce.byteLength > keyNonceBytes) keynonce = seed.subarray(0, keyNonceBytes);
-			if (keynonce.byteLength < keyNonceBytes) {
-				const y = new Uint8Array(keyNonceBytes);
-				y.set(keynonce);
-				keynonce = y;
-			}
+		let keynonce = seed;
+		if (keynonce.byteLength > keyNonceBytes) keynonce = seed.subarray(0, keyNonceBytes);
+		if (keynonce.byteLength < keyNonceBytes) {
+			const y = new Uint8Array(keyNonceBytes);
+			y.set(keynonce);
+			keynonce = y;
 		}
 		const key = keynonce.subarray(0, CHACHA_KEYSIZE);
 		const nonce = keynonce.subarray(CHACHA_KEYSIZE, CHACHA_KEYSIZE + CHACHA_IVSIZE);
diff --git a/src/utils/random/genrng.ts b/src/utils/random/genrng.ts
index 799308fe..fff2ee9e 100644
--- a/src/utils/random/genrng.ts
+++ b/src/utils/random/genrng.ts
@@ -3,10 +3,9 @@ import { FN_NATIVE, NULL, NUM } from '../../interpreter/value.js';
 import { textEncoder } from '../../const.js';
 import { SeedRandomWrapper } from './seedrandom.js';
 import { ChaCha20 } from './chacha20.js';
-import type { VNativeFn, VNull, Value } from '../../interpreter/value.js';
+import type { VNativeFn, VNum, VStr } from '../../interpreter/value.js';
 
-export function GenerateLegacyRandom(seed: Value | undefined) : VNativeFn | VNull {
-	if (!seed || seed.type !== 'num' && seed.type !== 'str') return NULL;
+export function GenerateLegacyRandom(seed: VNum | VStr): VNativeFn {
 	const rng = seedrandom(seed.value.toString());
 	return FN_NATIVE(([min, max]) => {
 		if (min && min.type === 'num' && max && max.type === 'num') {
@@ -16,8 +15,7 @@ export function GenerateLegacyRandom(seed: Value | undefined) : VNativeFn | VNul
 	});
 }
 
-export function GenerateRC4Random(seed: Value | undefined) : VNativeFn | VNull {
-	if (!seed || seed.type !== 'num' && seed.type !== 'str') return NULL;
+export function GenerateRC4Random(seed: VNum | VStr): VNativeFn {
 	const rng = new SeedRandomWrapper(seed.value);
 	return FN_NATIVE(([min, max]) => {
 		if (min && min.type === 'num' && max && max.type === 'num') {
@@ -28,13 +26,12 @@ export function GenerateRC4Random(seed: Value | undefined) : VNativeFn | VNull {
 	});
 }
 
-export async function GenerateChaCha20Random(seed: Value | undefined) : Promise<VNativeFn | VNull> {
-	if (!seed || seed.type !== 'num' && seed.type !== 'str' && seed.type !== 'null') return NULL;
-	let actualSeed : Uint8Array | undefined = undefined;
+export async function GenerateChaCha20Random(seed: VNum | VStr): Promise<VNativeFn> {
+	let actualSeed: Uint8Array;
 	if (seed.type === 'num')
 	{
 		actualSeed = new Uint8Array(await crypto.subtle.digest('SHA-384', new Uint8Array(new Float64Array([seed.value]))));
-	} else if (seed.type === 'str') {
+	} else {
 		actualSeed = new Uint8Array(await crypto.subtle.digest('SHA-384', new Uint8Array(textEncoder.encode(seed.value))));
 	}
 	const rng = new ChaCha20(actualSeed);
diff --git a/test/std.ts b/test/std.ts
index 3018a11f..22598d13 100644
--- a/test/std.ts
+++ b/test/std.ts
@@ -1,6 +1,7 @@
 import * as assert from 'assert';
-import { describe, test } from 'vitest';
+import { describe, expect, test } from 'vitest';
 import { utils } from '../src';
+import { AiScriptRuntimeError } from '../src/error';
 import { NUM, STR, NULL, ARR, OBJ, BOOL, TRUE, FALSE, ERROR ,FN_NATIVE } from '../src/interpreter/value';
 import { exe, eq } from './testutils';
 
@@ -156,6 +157,10 @@ describe('Math', () => {
 		`)
 		eq(res, ARR([BOOL(true), BOOL(true)]));
 	});
+
+	test.concurrent('gen_rng should reject when null is provided as a seed', async () => {
+		await expect(() => exe('Math:gen_rng(null)')).rejects.toThrow(AiScriptRuntimeError);
+	});
 });
 
 describe('Obj', () => {
diff --git a/unreleased/math-gen-rng-disallow-null-seed.md b/unreleased/math-gen-rng-disallow-null-seed.md
new file mode 100644
index 00000000..c224e27a
--- /dev/null
+++ b/unreleased/math-gen-rng-disallow-null-seed.md
@@ -0,0 +1 @@
+- Fix: `Math:gen_rng`の`seed`に`null`を与えてもエラーが発生しない問題を修正