authentication does not works on server

this is my code 
 
Dim str() As String

Dim role As System.Web.Security.RolePrincipal  
Dim countUserRoles As Integer                

role = CType(User, System.Web.Security.RolePrincipal) 

str = role.GetRoles()     

 
 the line role = CType(User, System.Web.Security.RolePrincipal)  ever throws an exception :
Unable to cast object of type 'System.Security.Principal.GenericPrincipal' to
type 'System.Web.Security.RolePrincipal'.
 
In my local enviroment it works fine but on server this error always happens 
 
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