JavaScriptSerializer.Deserialize Method can have 2 arguments.It should be like below.So check the datatypes of your method.
JavaScriptSerializer.Deserialize Method (String, Type);
public Object Deserialize(
string input,
Type targetType
)
Parameters
input
Type: System.String
The JSON string to deserialize.
targetType
Type: System.Type
The type of the resulting object.
Please read this for more info:
JavaScriptSerializer.Deserialize Method (String, Type)