How to emit default(TimeSpan) as the default value of an optional parameter

I want to emit dynamic method which is exactly the same as below:

void Foo(TimeSpan ts = default(TimeSpan))

By using ildasm, i can see it has been complied as nullref.

However from what i can get, if i want to achieve the same by Emit code, the method i can call named ParameterBuilder.SetConstant, and it will throw exception when the optional value type is TimeSpan.

I even decompiled SetConstant method, it handles DateTime explicitly(but not TimeSpan). Nullref is also not acceptable. From that code, it seems there's no way to set default(TimeSpan) as a default value.

Can anyone help?