Example of NSubstitute type mismatch exception for nested returns.

gistfile1.txt

Can not return value of type IFooProxy for IFoo.set_SomeString (expected type Void).

Make sure you called Returns() after calling your substitute (for example: mySub.SomeMethod().Returns(value)),
and that you are not configuring other substitutes within Returns() (for example, avoid this: mySub.SomeMethod().Returns(ConfigOtherSub())).

If you substituted for a class rather than an interface, check that the call to your substitute was on a virtual/abstract member.
Return values cannot be configured for non-virtual/non-abstract members.

Correct use:
  mySub.SomeMethod().Returns(returnValue);

Potentially problematic use:
	mySub.SomeMethod().Returns(ConfigOtherSub());
Instead try:
	var returnValue = ConfigOtherSub();
	mySub.SomeMethod().Returns(returnValue);