Add manual DbSet to Partial class DbContext EntityFramework

I get an old project where I need to add a new functionnality. Edmx was generated a while ago and I don't want to edit it or generate it back. My idea is simply to add a new partial class with my two new DbSet:

public partial class MyContextClass
{
    public virtual DbSet<Labels> Labels { get; set; }
    public virtual DbSet<LabelsWeight> LabelsWeights { get; set; }
}

My edmx generate the original MyContextClass:

public partial class MyContextClass : DbContext
{ .... }

Here is my Business object:

public class Labels
{
    public int Id { get; set; }
    public string Type { get; set; }
    public string Label { get; set; }
    public Nullable<bool> Active { get; set; }
    public string Operation { get; set; }
    public string Value { get; set; }
}

In DB, I have:

CREATE TABLE [dbo].[Labels](
[Id] [int] IDENTITY(1,1) NOT NULL,
[Type] [nvarchar](50) NOT NULL,
[Label] [nvarchar](100) NOT NULL,
[Active] bit NULL,
[Operation] [nvarchar](5) NULL,
[Value] [nvarchar](50) NULL,
 CONSTRAINT [PK.Labels] PRIMARY KEY CLUSTERED 
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF,            
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

But I get error:

The entity type Labels is not part of the model for the current context.

Is it something that I missed ?