Update a field in the database

I have the following table in informix:

CREATE TABLE eh_user (
    user_key serial NOT NULL,
    borrado boolean DEFAULT f NOT NULL,
    fecha_alta datetime DEFAULT current NOT NULL,
    propietario varchar,
    fecha_modificacion datetime,
    version int DEFAULT 0 NOT NULL,
    birth_date datetime,
    collegiate_number varchar,
    email varchar,
    inactive boolean,
    inactive_date datetime,
    inactive_reason varchar,
    login varchar(150) NOT NULL,
    name varchar(250) NOT NULL,
    nif varchar,
    password varchar,
    surname1 varchar(250) NOT NULL,
    surname2 varchar(250),
    surname2_required varchar,
    telefon_number varchar,
    locale_key int NOT NULL,
    position_key int NOT NULL,
    sex_type_key int NOT NULL,
    PRIMARY KEY (user_key) CONSTRAINT u227_47,
    FOREIGN KEY (locale_key) REFERENCES locale(locale_key) 
    FOREIGN KEY (position_key) REFERENCES type_master(type_master_key) 
    FOREIGN KEY (sex_type_key) REFERENCES type_master(type_master_key) 
);

As you can see, inactive is a field of type boolean, so every time I try to update that field in a record, or insert a row by giving it a value true or false I get the following error :

SQL Error [IX000]: Column (true) not found in any table in the query (or SLV is undefined).
  Column (true) not found in any table in the query (or SLV is undefined).
  Column (true) not found in any table in the query (or SLV is undefined).

in this case with a simple update.