I'm creating a new type with a new field. If the new field is of type "text", there's no problem. If the new field is of type "integer", here's the warning :
FieldException: Attempt to create a field of unknown type integer. in field_create_field() (line 106 of /home/scanagen/public_html/dev/modules/field/field.crud.inc).
Here's the code working, placed in _mymodulename_installed_instances() and in _mymodulename_installed_fields():