Composite Key validation not working: Laravel 5.2

What I tried so far ?

My Request class is below

class SubCategoryRequest extends Request
{
    public function authorize()
    {
        if(\Auth::user()->RoleID == \App\Enumeration\Role\RoleType::Admin) {
            return true;
        }
        return false;
    }

    public function rules()
    {
        $rules = [];
        $rules['SubCategory'] = 'required|max:25|min:5|unique:tblsubcategory,CategoryID';
        $rules['CategoryID'] = 'required';
        return $rules;
    }

    public function response(array $errors){
        return \Redirect::back()->withErrors($errors)->withInput();
    }
}

Below is the Database Table Schema

CREATE TABLE `tblsubcategory` (
  `SubCategoryID` int(11) NOT NULL,
  `SubCategory` varchar(25) NOT NULL,
  `CategoryID` int(11) NOT NULL,
  `IsActive` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

ALTER TABLE `tblsubcategory`
  MODIFY `SubCategoryID` int(11) NOT NULL AUTO_INCREMENT;

Below is Unique Key Constraint

ALTER TABLE `tblsubcategory`
  ADD PRIMARY KEY (`SubCategoryID`),
  ADD UNIQUE KEY `UK_tblSubCategory_SubCategory_CategoryID` (`CategoryID`,`SubCategory`);

Question

Am I missing something ?

via Chebli Mohamed