Custom free text field cannot have value "-1"

I am trying to use a custom free text field to store the error number of an exception.

Unfortunately, if I try to store the string value "-1", it is as if I typed the empty string, and it deletes any existing value of that field. This affects the SOAP API too.

I can't use a numeric field as this field is used for non-numeric strings elsewhere.

Apologies if this is a duplicate but it's somewhat hard to search for.

Thanks.