How do I fake System.Data.SqlTypes.SqlString?

How do I get around this error message?

*** Faking behavior on struct System.Data.SqlTypes.SqlString is not supported in the following cases:
1. Faking behavior on more than one struct of the same type.
2. Faking behavior on more than one method on the same struct.

Here is my code
var format = Isolate.Fake.Instance<SqlString>(Members.ReturnRecursiveFakes);
Isolate.WhenCalled(() => format.IsNull).WillReturn(true);