Cannot create foreign key

I want to create a foreign key that is string from table "stocks" to "rfids". The two tables are shown below. stocks table:

Schema::create('stocks', function (Blueprint $table) {            

        $table->increments('tag_no');                       
        $table->string('stock_type');

        $table->string('rfid');
        $table->foreign('rfid')->references('RFID_UID')->on('rfids');

        $table->integer('user_id')->unsigned();
        $table->foreign('user_id')->references('id')->on('users');

        $table->timestamps();            
    });

rfids table:

Schema::create('rfids', function (Blueprint $table) {            

        $table->increments('id');
        $table->string('RFID_UID');

        $table->timestamps();
    });

When i use php artisan migrate it shows error.

SQLSTATE[HY000]: General error: 1005 Can't create table hardware.#sql-81c_c8 (errno: 150 "Foreign key constraint is incorrectly formed") (SQL: alter table stocks add constraint stocks_rfid_foreign foreign key (rfid) references rfids (RFID_UID))

Someone help me pls!

via Chebli Mohamed