diff --git a/server/src/database/database.module.spec.ts b/server/src/database/database.module.spec.ts
index 237bc13..ac2747a 100644
--- a/server/src/database/database.module.spec.ts
+++ b/server/src/database/database.module.spec.ts
@@ -1,7 +1,7 @@
 import { Test } from '@nestjs/testing';
 import { ConfigModule as NestConfigModule } from '@nestjs/config';
 import configuration from '../config/configuration';
-import { DatabaseModule } from './database.module';
+import { DatabaseModule, assertDatabaseUrlAllowed } from './database.module';
 import { PrismaClient } from '../../generated/prisma/client';
 
 /**
@@ -55,4 +55,39 @@ describe('DatabaseModule', () => {
     expect(typeof client.$disconnect).toBe('function');
     await moduleRef.close();
   });
+
+  describe('assertDatabaseUrlAllowed', () => {
+    it('throws when URL is empty or undefined', () => {
+      expect(() => assertDatabaseUrlAllowed('', 'development')).toThrow(
+        /DATABASE_URL is not configured/,
+      );
+      expect(() => assertDatabaseUrlAllowed(undefined, 'development')).toThrow(
+        /DATABASE_URL is not configured/,
+      );
+    });
+
+    it('throws when URL is file: in production', () => {
+      expect(() =>
+        assertDatabaseUrlAllowed('file:./dev.db', 'production'),
+      ).toThrow(/not allowed when NODE_ENV=production/);
+    });
+
+    it('does not throw when URL is file: in development or test', () => {
+      expect(() =>
+        assertDatabaseUrlAllowed('file:./dev.db', 'development'),
+      ).not.toThrow();
+      expect(() =>
+        assertDatabaseUrlAllowed('file:./dev.db', 'test'),
+      ).not.toThrow();
+    });
+
+    it('does not throw for non-file database URL in production', () => {
+      expect(() =>
+        assertDatabaseUrlAllowed(
+          'postgresql://user:pass@localhost:5432/db',
+          'production',
+        ),
+      ).not.toThrow();
+    });
+  });
 });
diff --git a/server/src/database/database.module.ts b/server/src/database/database.module.ts
index 31d2316..5f4c609 100644
--- a/server/src/database/database.module.ts
+++ b/server/src/database/database.module.ts
@@ -8,7 +8,15 @@ import { PrismaPg } from '@prisma/adapter-pg';
  * production, so a misconfigured deploy fails fast at startup instead of
  * silently running against a throwaway local file.
  */
-export function assertDatabaseUrlAllowed(url: string, nodeEnv: string): void {
+export function assertDatabaseUrlAllowed(
+  url: string | undefined,
+  nodeEnv: string,
+): void {
+  if (!url) {
+    throw new Error(
+      'DATABASE_URL is not configured or resolved to an empty value.',
+    );
+  }
   if (nodeEnv === 'production' && url.startsWith('file:')) {
     throw new Error(
       `DATABASE_URL resolves to a local file ("${url}"), which is not allowed when NODE_ENV=production. Set DATABASE_URL to a real database connection string.`,