title: Value cannot be null. Parameter name: source" Entity Framework Code First
body: <p>Hi I'm try to use EF with a code first workflow against an existing database but I am getting the error:</p>
  
  <p>{"Value cannot be null.
  Parameter name: source"}</p>
  
  <p>when using the following line:</p>
  
  <pre><code>    Dim ctx = New MVCTest1
      Dim t = ctx.Companies.ToList()
  </code></pre>
  
  <p>My POCO classes are included below:</p>
  
  <p>I have a standard conn string called MVCTest1 which points at SQL Server. I think EF is using that as I can see it in connection string of the dbContext. Does the conn string need to be in a special format. I also noticed the onmodelcreated isn't being called. I'm not sure if that's relevant. Very new to this so I suspect it's something obvious Cheers, Chris</p>
  
  <pre><code>Imports System.Data.Entity
  
  Public Class Company
      Public CompanyID As Integer
      Public CompanyName As String
      Public Overridable Property Contacts As ICollection(Of Contact)
  End Class
  
  Public Class Contact
      Public ContactID As Integer
      Public FirstName As String
      Public LastName As String
      Public Overridable Property Company As Company
  End Class
  
  Public Class MVCTest1
      Inherits DbContext
  
      Public Contacts As DbSet(Of Contact)
      Public Companies As DbSet(Of Company)
  
      Protected Overrides Sub OnModelCreating(ByVal modelBuilder As System.Data.Entity.DbModelBuilder)
  
  ---Not being called
  
  
  
  
    End Sub
  
  
  End Class
  </code></pre>