Implicit conversion from data type sql_variant to varchar is not allowed

I'm getting this error when calling a stored procedure from .NET Entity Framework using Dapper.

Implicit conversion from data type sql_variant to varchar is not allowed.

The stored procedure parameter is @Name nvarchar(50) = NULL.

So ideally, it should accept a null value. When I call the stored procedure and pass @Name = null from .NET, I get gives this error.

This is the stored procedure:

CREATE PROCEDURE [dbo].[Member_Add]
    @Id NVARCHAR(50)  = NULL,
    @Name NVARCHAR(50)  = NULL
AS
BEGIN
    INSERT INTO [dbo].Member (Id, Name)
    VALUES (@Id, @Name)
END