Entity Framework error during update: field is required - entity-framework

Question

I'm using EF 6.2.0, Code First from Database, in a .net 4.5.2 environment.
I have this DbSet:
<Table("Firebird.PLANT4")>
Partial Public Class PLANT4
<Key>
<DatabaseGenerated(DatabaseGeneratedOption.None)>
Public Property ID_PLANT4 As Integer
Public Property STATUS As Integer
<Required>
<StringLength(20)>
Public Property DESCRIPTION1 As String
Public Property COUNTER As Long
Public Property RESET_COUNTER As Integer
End Class
When I execute this code:
Using dbContext As New DbModel
dbContext.Database.Connection.ConnectionString = GetFbConnectionString()
dbContext.Database.Connection.Open()
Dim plant As PLANT4 = dbContext.PLANT4.Find(1)
plant.RESET_COUNTER = 1
dbContext.SaveChanges()
End Using
I get the error: "DESCRIPTION1 field is required".
The exception is throwing during SaveChanges.
I can't understand where the problem is, as if I watch "plant" in debug, all fields are there (ID_PLANT4 = 1 is an existing row), and DESCRIPTION1 in particular is not Nothing.
I can simply remove the "Required" attribute and it works, but the attribute is a consequence of the db column not allowing nulls, so I don't think this is the right way to go.
I can even add this line of code, just after the "Using" statement:
dbContext.Configuration.ValidateOnSaveEnabled = False
and it works, but again I don't think this is the right way to go.
What is the reason of this behavior?

Answer 1

Eventually I find that the problem is: field DESCRIPTION1 is populate by default with 20 spaces. It is not null, but it is a string formed only by spaces. For an unknown reason, during validation this string is treated by EF like null, and an exception is thrown because it's a required field.
"Required" attribute is not needed, but I generate my POCO classes by "Code First from Database", so if a VARCHAR field is declared as "not null" it is automatically generated with the "Required" attribute. Now I think it's better allowing nulls for VARCHAR columns.