The problem is in ParameterBuilder.SetConstant, not Activator.CreateInstance. What is going on? Is there any other possible way to create a nullable when I don't know the generic parameters until run-time? Heck, if I copy & paste nullable from DotNetReflector (and remove TypeDependencyAttribute, TargetPatchingOptOutAttribute, and ThrowHelper calls since I don't have access to those), it shows False above. A struct created in C#, even a generic one, is created fine. For example, simply executing: object Test = Activator.CreateInstance(typeof(Nullable)) Unfortunately, whenever I use Activator.CreateInstance(NullableType) (where NullableType is a created type from user input) I get null as the return value. The problem is that I need to create an instance of user selected nullable within the compiler and set it as the constant. I'm encountering a problem when I try to take a nullable value class and make it the default value of a parameter. I'm creating a system that turns a small script into a dll.

Make activator