Aceita userName existente

Quando eu coloco um usuario existente ele mostra a mensagem 'user available'. Segue o codigo:

user-not-taken.validator.service.ts

checkUserNameTaken(){
        return (control: AbstractControl) => {
            return control.
                valueChanges.
                pipe(debounceTime(300)).
                pipe(switchMap(userName => 
                    this.signupService.checkUserNameTaken(userName)
                )).
                pipe(map(isTaken =>
                    isTaken ? {userNameTaken: true } : null
                )).
                pipe(first());
        }
    }

signup.service.ts

checkUserNameTaken(userName: string){
        return this.http.get(API_URL + '/user/exists/'+userName);
    }

signup.component.ts

 userName: ['', 
          [
            Validators.required, 
            lowerCaseValidator,
            Validators.minLength(2),
            Validators.maxLength(30)
          ],
          this.userNotTakenValidatorService.checkUserNameTaken()
        ],

signup.component.html

<ap-vmessage 
            text="User name already taken!"
            *ngIf="signupForm.get('userName').errors?.userNameTaken"
        ></ap-vmessage>
        <small *ngIf="signupForm.get('userName').valid" class="text-success">User available!</small>

OBS: as outras validações funcionam normalmente