Conversion of nullable double variable to String in VB.NET

Recently I came across a weird behaviour of a nullable double. As I need to turn a nullable variable into string with precisely set digits I am using method .ToString(format). When I apply this method on the aforementioned nullable variable instead of getting converted number as I need, I get only the first digit of this number (example is given bellow). Of course I can cast the nullable to double and it solves it, but why is it happening though. Thank you very much for your input.

Example:

'Nullable Double
Dim value As Double? = 867
Dim convertedValue As String = value.ToString("0.00000")
Console.WriteLine(convertedValue)
'Output: 8

'Not nullable Double
Dim value As Double = 867
Dim convertedValue As String = value.ToString("0.00000")
Console.WriteLine(convertedValue)
'Output: 867.00000

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