diff --git a/projects/ngneat/cashew/src/lib/cache-context.ts b/projects/ngneat/cashew/src/lib/cache-context.ts
index 05efd6a..321c987 100644
--- a/projects/ngneat/cashew/src/lib/cache-context.ts
+++ b/projects/ngneat/cashew/src/lib/cache-context.ts
@@ -3,10 +3,12 @@ import { EMPTY, Observable } from 'rxjs';
 import { CacheBucket } from './cache-bucket';
 import { HttpCacheConfig } from './cache-config';
 
+export type CacheContextKey = string | number;
+
 export interface ContextOptions {
   cache?: boolean;
   ttl?: number;
-  key?: string | ((request: HttpRequest<any>) => string);
+  key?: CacheContextKey | ((request: HttpRequest<any>) => CacheContextKey);
   bucket?: CacheBucket;
   version?: string;
   clearCachePredicate?<T>(
diff --git a/projects/ngneat/cashew/src/lib/key-serializer.ts b/projects/ngneat/cashew/src/lib/key-serializer.ts
index ab82500..1ad5459 100644
--- a/projects/ngneat/cashew/src/lib/key-serializer.ts
+++ b/projects/ngneat/cashew/src/lib/key-serializer.ts
@@ -10,7 +10,7 @@ export class DefaultKeySerializer extends KeySerializer {
     const { key } = context;
 
     if (key) {
-      return typeof key === 'function' ? key(request) : key;
+      return (typeof key === 'function' ? key(request) : key).toString();
     }
 
     return request.urlWithParams;
diff --git a/projects/ngneat/cashew/src/lib/specs/key-serializer.spec.ts b/projects/ngneat/cashew/src/lib/specs/key-serializer.spec.ts
new file mode 100644
index 0000000..3bdfa5d
--- /dev/null
+++ b/projects/ngneat/cashew/src/lib/specs/key-serializer.spec.ts
@@ -0,0 +1,35 @@
+import { DefaultKeySerializer } from '../key-serializer';
+import { httpRequest } from './mocks';
+
+describe('key serializer', () => {
+  it('serializes to string using the given number', () => {
+    const serializer = new DefaultKeySerializer();
+    const key = serializer.serialize(null, { key: 2 });
+    expect(key).toEqual('2');
+  });
+
+  it('serializes to string using the given string', () => {
+    const serializer = new DefaultKeySerializer();
+    const key = serializer.serialize(null, { key: 'hello' });
+    expect(key).toEqual('hello');
+  });
+
+  it('serializes to string using the given number producing function', () => {
+    const serializer = new DefaultKeySerializer();
+    const key = serializer.serialize(null, { key: () => 2 + 2 });
+    expect(key).toEqual('4');
+  });
+
+  it('serializes to string using the given string producing function', () => {
+    const serializer = new DefaultKeySerializer();
+    const key = serializer.serialize(null, { key: () => 'hello' + 'world' });
+    expect(key).toEqual('helloworld');
+  });
+
+  it('serializes to full url including parameters if key is absent', () => {
+    const serializer = new DefaultKeySerializer();
+    const request = httpRequest();
+    const key = serializer.serialize(request, {});
+    expect(key).toEqual(request.urlWithParams);
+  });
+});