Abstract Class Mappings
If you're running into this error in NHibernate:
System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values.
Parameter name: Attempting to parse a null value into an sql string.
Examine your mappings for abstract classes that aren't implemented, or classes that are marked as abstract in the mappings but not in the c# file. I was refactoring out some abstract classes in the middle and forgot to delete the abstract="true" line in the mapping.