By default a business object does not allow NULL values. To add NULL value support you need to do two things.1. Set the AlloNullValuesOnNewRow to True on the BO itself.
2. Next, in the BO Mapper, you can specify NULL support. There are two options, Generic Nullable and Return Alternate Value on NULL. This does not affect the data on the server, but adds support to the strong-typed properties so the controls can interact without failing. The most common option is Return Alternate on NULL. When using this method, simply put the desired return value on in the text spacea below. If you want to return an empty string, simply put the following value in the provided field: ""
This should resolve this error.