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

Question

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

Code

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

Error

failed : Cannot add Foreign key Constraint

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