Can't modify an existing SPF record

[Roberto Carna]

Dear, from my webmin interface for BIND9, I try to add an additional
allowed sender host to our SPF record, but I get the following error:

Failed to save record : 'relay.company.com' is not a valid host to
allow sending from

What does this mean? Do I have to consider some important thing I'm forgetting ?

relay.company.com is already defined in our public DNS, and it has a
reverse record too.

if I add this record by hand, it's not replicated to the DNS slaves.

Thanks in advance!!!