Field not saved when value is -1

Hello,
I have a custom field (text, single line).
I have an issue type whose creation and edit/view screens use the custom field.
When I create a issue of that type and assign the value -1 to the custom field, this value is not saved (it is saved as empty). I have tried with any other string and it is saved correctly.
This happens whenever I create an issue both through the UI and through the JSON REST API. And also when I edit the issue and assign the value -1 to the field.