Every time you change the type of field, you must read the database‘s schema.
This article explains how to do that.
See the steps below.
1: Open the application.
2: Right-click and select “Developer.“
3: Double click “Database Manager.“
4: Click “Actions“ from the menu and select “Read schema.“
5: After the scheme‘s reading, close the application.
This action creates the file data.db in the folder “RESOURCES.“
For more help, please see http://www.youtube.com/watch?v=U3Oectp6bzI