diff --git a/server/src/services/partner.service.spec.ts b/server/src/services/partner.service.spec.ts
index 029462a865..b947f3795a 100644
--- a/server/src/services/partner.service.spec.ts
+++ b/server/src/services/partner.service.spec.ts
@@ -54,6 +54,7 @@ describe(PartnerService.name, () => {
       const auth = AuthFactory.create({ id: user1.id });
 
       mocks.partner.get.mockResolvedValue(void 0);
+      mocks.user.get.mockResolvedValue(user2);
       mocks.partner.create.mockResolvedValue(getForPartner(partner));
 
       await expect(sut.create(auth, { sharedWithId: user2.id })).resolves.toBeDefined();
@@ -76,6 +77,19 @@ describe(PartnerService.name, () => {
 
       expect(mocks.partner.create).not.toHaveBeenCalled();
     });
+
+    it('should throw an error when sharedWithId does not resolve to an existing (non-deleted) user', async () => {
+      const user1 = UserFactory.create();
+      const user2 = UserFactory.create();
+      const auth = AuthFactory.create({ id: user1.id });
+
+      mocks.partner.get.mockResolvedValue(void 0);
+      mocks.user.get.mockResolvedValue(void 0);
+
+      await expect(sut.create(auth, { sharedWithId: user2.id })).rejects.toBeInstanceOf(BadRequestException);
+
+      expect(mocks.partner.create).not.toHaveBeenCalled();
+    });
   });
 
   describe('remove', () => {
diff --git a/server/src/services/partner.service.ts b/server/src/services/partner.service.ts
index cc950edb5b..26d7701077 100644
--- a/server/src/services/partner.service.ts
+++ b/server/src/services/partner.service.ts
@@ -16,6 +16,12 @@ export class PartnerService extends BaseService {
       throw new BadRequestException(`Partner already exists`);
     }
 
+    const user = await this.userRepository.get(sharedWithId, {});
+    if (!user) {
+      this.logger.debug('Partner creation failed: user not found');
+      throw new BadRequestException('Invalid user');
+    }
+
     const partner = await this.partnerRepository.create(partnerId);
     return this.mapPartner(partner, PartnerDirection.SharedBy);
   }