Create CONSTRAINT FOREINGN KEY with primary key VARCHAR type from another MYSQL table

I have two tables one call _hardware_ and another call _responsavel_ . In _responsavel_ I have a Primary Key called NIP of type VARCHAR(8) . I created a field called NIP_RESP also VARCHAR(8) , I'd like to reference the NIP column of the _responsavel_ table as Foreign Key in the NIP_RESP column of the _hardware table.

Code

ALTER TABLE hardware
    ADD CONSTRAINT 'fk_NIP_RESP' FOREIGN KEY ( NIP_RESP ) REFERENCES responsavel ( NIP );

Error

  

failed: Can not add foreign key constraint

It should be noted that both are NOT NULL DEFAULT VALUE 99999999