Why type of errors that could occur when passing Dictionary?

Question

Hi,
I have a parameter that takes Dictionary<string, string> as an argument. If i want to validate the input parameters, what types of checking that i can perform?
As far as know, the caller cannot pass Another Collection object other than Dictionary (ie. cant pass ArrayList, List<T>....) becuase the compiler raise errors and in the Key, value check type checking is not neccessary becuase any type can be casted to string type. Therfore what type of checking that i can perform on the passed parameter?
TY in Advance