Type Conversion Error when i insert empty textbox

create table products(product nvarchar(max),
price numeric(18,2),
discount numeric(18,2) default 0)

CREATE PROCEDURE sp1

@p1 NVARCHAR(MAX),

@p2 NUMERIC(18,2),

@p3 NUMERIC(18,2) default 0
AS
BEGIN
INSERT INTO products  VALUES (@p1 ,@p2 ,@p3 )
End
GO

com = New SqlCommand("sp1", con)
        com.CommandType = CommandType.StoredProcedure
        Try
            com.Parameters.AddWithValue("@p1", TextBox1.Text)
            com.Parameters.AddWithValue("@p2", TextBox2.Text)
            com.Parameters.AddWithValue("@p3", TextBox3.Text)
            com.ExecuteNonQuery()
            MsgBox("Record Added....")

        Catch ex As Exception
            MsgBox(ex.Message)
        End Try