Dotnet type StringBuilder not working as step parameter type

Hello,

i created some functions in C# that i want to use in TestStand.

One of them looks like this:
public void myfunction(int a, ref int b, StringBuilder c)
{
//somecode
}

When i insert this in teststand 2016 and run the step i get a Run-Time Error:
An error occurred while calling .NET member 'myfunction':
Object of type 'System.__ComObject' cannot be converted to type 'System.Text.StringBuilder'.

Errorcode: -17500; Operation Failed.

Question: Is it not possible to use type StringBuilder?

Thx