diff --git a/fluxer_api/src/api/database/CassandraParamGuard.test.ts b/fluxer_api/src/api/database/CassandraParamGuard.test.ts
new file mode 100644
index 000000000..714af0f3a
--- /dev/null
+++ b/fluxer_api/src/api/database/CassandraParamGuard.test.ts
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+import {describe, expect, it} from 'vitest';
+import {assertNoUndefinedParams} from './CassandraTypes';
+
+function messageFor(path: string): string {
+	return `Undefined value at "${path}". This project forbids undefined in Cassandra params; use null explicitly or omit the column via PATCH.`;
+}
+
+function guardError(params: Record<string, unknown>): string {
+	try {
+		assertNoUndefinedParams(params);
+	} catch (error) {
+		return (error as Error).message;
+	}
+	throw new Error('Expected assertNoUndefinedParams to throw');
+}
+
+describe('assertNoUndefinedParams', () => {
+	it('accepts a realistic message row without throwing', () => {
+		expect(() =>
+			assertNoUndefinedParams({
+				message_id: 123n,
+				channel_id: 456n,
+				content: 'hello',
+				edited_at: null,
+				pinned: false,
+				created_at: new Date(0),
+				blob: Buffer.from('x'),
+				mention_users: [1n, 2n, 3n],
+				nsfw_emojis: new Set(['a', 'b']),
+				reactions: new Map([['a', 1]]),
+				embeds: {title: 'a', fields: [{name: 'n', value: 'v'}], footer: {text: null}},
+			}),
+		).not.toThrow();
+	});
+
+	it('reports the dotted path of a top level undefined param', () => {
+		expect(guardError({user_id: undefined})).toBe(messageFor(':user_id'));
+	});
+
+	it('reports the dotted path of an undefined array element', () => {
+		expect(guardError({mention_users: [1n, undefined, 3n]})).toBe(messageFor(':mention_users[1]'));
+	});
+
+	it('reports the dotted path of an undefined set member', () => {
+		expect(guardError({nsfw_emojis: new Set(['a', undefined])})).toBe(messageFor(':nsfw_emojis{set:1}'));
+	});
+
+	it('reports the dotted path of an undefined map key', () => {
+		expect(guardError({reactions: new Map([['a', 1] as const, [undefined, 2] as const])})).toBe(
+			messageFor(':reactions{mapKey:1}'),
+		);
+	});
+
+	it('reports the dotted path of an undefined map value', () => {
+		expect(
+			guardError({
+				reactions: new Map([
+					['a', 1],
+					['b', undefined],
+				]),
+			}),
+		).toBe(messageFor(':reactions{mapVal:1}'));
+	});
+
+	it('reports the dotted path of an undefined nested object property', () => {
+		expect(guardError({embeds: {footer: {icon_url: undefined}}})).toBe(messageFor(':embeds.footer.icon_url'));
+	});
+
+	it('reports the dotted path of an undefined value nested through mixed containers', () => {
+		expect(guardError({embeds: [{fields: new Set([new Map([['inline', undefined]])])}]})).toBe(
+			messageFor(':embeds[0].fields{set:0}{mapVal:0}'),
+		);
+	});
+
+	it('reports the first undefined in parameter order', () => {
+		expect(guardError({a: 1, b: [undefined], c: undefined})).toBe(messageFor(':b[0]'));
+	});
+});
diff --git a/fluxer_api/src/api/database/CassandraTypes.ts b/fluxer_api/src/api/database/CassandraTypes.ts
index 5ccc35b80..3ee5e3fdd 100644
--- a/fluxer_api/src/api/database/CassandraTypes.ts
+++ b/fluxer_api/src/api/database/CassandraTypes.ts
@@ -254,6 +254,41 @@ export function isUnsafePreparedStatement(query: string): boolean {
 	return tokens.length >= 2 && tokens[0].toLowerCase() === 'select' && tokens[1] === '*';
 }
 
+function hasUndefinedDeep(value: unknown): boolean {
+	if (value === undefined) return true;
+	if (value === null) return false;
+	const t = typeof value;
+	if (t === 'string' || t === 'number' || t === 'bigint' || t === 'boolean') return false;
+	if (value instanceof Date) return false;
+	if (value instanceof Buffer) return false;
+	if (t === 'object' && value.constructor?.name === 'LocalDate') return false;
+	if (Array.isArray(value)) {
+		for (let i = 0; i < value.length; i++) {
+			if (hasUndefinedDeep(value[i])) return true;
+		}
+		return false;
+	}
+	if (value instanceof Set) {
+		for (const v of value.values()) {
+			if (hasUndefinedDeep(v)) return true;
+		}
+		return false;
+	}
+	if (value instanceof Map) {
+		for (const [k, v] of value.entries()) {
+			if (hasUndefinedDeep(k) || hasUndefinedDeep(v)) return true;
+		}
+		return false;
+	}
+	if (t === 'object') {
+		const keys = Object.keys(value as Record<string, unknown>);
+		for (let i = 0; i < keys.length; i++) {
+			if (hasUndefinedDeep((value as Record<string, unknown>)[keys[i]!])) return true;
+		}
+	}
+	return false;
+}
+
 function assertNoUndefinedDeep(value: unknown, path: string): void {
 	if (value === undefined) {
 		throw new Error(
@@ -297,8 +332,13 @@ function assertNoUndefinedDeep(value: unknown, path: string): void {
 }
 
 export function assertNoUndefinedParams(params: Record<string, unknown>): void {
-	for (const [k, v] of Object.entries(params)) {
-		assertNoUndefinedDeep(v, `:${k}`);
+	const keys = Object.keys(params);
+	for (let i = 0; i < keys.length; i++) {
+		const k = keys[i]!;
+		const v = params[k];
+		if (hasUndefinedDeep(v)) {
+			assertNoUndefinedDeep(v, `:${k}`);
+		}
 	}
 }