NullReferenceException on Dictionary.Add with dictionary variable not Null, key not Null and no multithreading

There have been a number of similar questions here, but none explains what is happening in my case; so here goes.

I have the following (simplified) piece of code:

' row is a System.Data.DataRow
' _typeProperties is a Dictionary(Of String, PropertyInfo)

Dim data As New Dictionary(Of String, Object)
Dim elem As KeyValuePair(Of String, PropertyInfo)
Dim value As Object = Nothing
Try
    For Each elem In _typeProperties
        value = row.Item(elem.Key)
        data.Add(elem.Key, value)       ' NullReferenceException here
    Next
Catch ex As Exception When MyExceptionFilter(ex, data, elem, value)
End Try

Sometimes I get a NullReferenceException on the indicated line. This exception is exceedingly rare and I cannot reproduce it at will. However, I can modify my application, send it out to a customer and sure enough, after some days it will reproduce on its own.

The call stack is not very helpful:

StackTrace: 
  XXX.RowToType(DataRow row) in C:\XXX.vb:line 645.

Moreover, as you can see I included an exception filter in the Catch block. In there I write a minidump (with the call stack of the exception intact). Here is the relevant portion of the call stack revealed by the minidump:

  ...
  App.exe!MyExceptionFilter( ex,  data,  elem,  value) Line 627
  App.exe!RowToType( row) Line 647 + 0x1f bytes
  [External Code]   
  App.exe!RowToType(System.Data.DataRow row) Line 645 + 0x112 bytes
  App.exe!SomeClass.get_Item(Integer index) Line 1141 + 0xe bytes   
  user32.dll!_InternalCallWinProc@20()  + 0x23 bytes    
  user32.dll!_UserCallWinProcCheckWow@32()  + 0x693 bytes   
  ...

The exception happens somewhere in the [External Code] block; then the filter of the Catch block gets executed (line 2 & line 1).

At the moment of the exception, these are the values for the three involved variables:

  data: Not Nothing; 
  elem: Not Nothing; 
  elem.Value: Not Nothing (Int32 ID)
  elem.Key: Not Nothing
  value: Nothing

So there appears to be absolutely no reason for data.Add to throw a NullReferenceException.

As some people suggested in other questions, there might be some threading issue. However, the dictionary to which I am writing can by definition only be visible to one thread. (And to be sure, I also checked the minidump to make sure that no thread is executing the same code.)

I might just silently ignore this exception, but I'd rather figure this out.