Create a foreign key with two varchar fields

I'm finding it very difficult to create a foreign key between two tables, whose type is varchar :

ALTER TABLE netmap ADD CONSTRAINT fk_IP FOREIGN KEY ( IP ) REFERENCES ipvalidos (IPS);

The error is:

  

Failed to execute SQL: SQL ALTER TABLE netmap ADD CONSTRAINT fk_IP FOREIGN KEY (IP) REFERENCES ipvalidos (IPS) failed: Can not add foreign key constraint

Before trying to foreign key , I gave describe , which looks like this:

ipvalidos

Field   Type          Null  Key     Default     Extra
IPS     varchar(255)  NO    PRI

netmap

Field   Type            Null    Key     Default     Extra
IP      varchar(15)     NO      MUL     
MAC     varchar(17)     NO      PRI     
MASK    varchar(15)     NO  
NETID   varchar(15)     NO      MUL     

I need to make a comparison between these tables, but I do not know where to start.