DataColumn has gone

I'm trying to add a column into a dataset and copy a part of an other datarow into the new made column like this:

VB Code:

Dim DsTemp As DataSet
        Dim myConnection As Odbc.OdbcConnection
        Dim ConnectieString As String
        Dim myDataRowsCommandBuilder As Odbc.OdbcCommandBuilder
 
 
        myConnection = New Odbc.OdbcConnection(GetDatabaseConn) 'get's the connectionstring from an other public place
        ConnectieString = "SELECT * FROM Humans"        
        mySqlDataAdapter = New Odbc.OdbcDataAdapter(ConnectieString, myConnection)
        DsTemp = New DataSet
        myDataRowsCommandBuilder = New Odbc.OdbcCommandBuilder(mySqlDataAdapter)
        mySqlDataAdapter.MissingSchemaAction = MissingSchemaAction.AddWithKey
        mySqlDataAdapter.Fill(DsTemp, "Humans")
        myConnection.Close()
 
        Dim Drow As DataRow
 
        If DsTemp.Tables(0).Columns("Activity") Is Nothing Then
            Dim DC As DataColumn = New DataColumn("Activity")
            DC.DataType = System.Type.GetType("System.Int32")
            DsTemp.Tables(0).Columns.Add(DC)
        End If
 
        For Each Drow In DsTemp.Tables(0).Rows
            If Len(Drow!projectnr) > 10 Then
                Drow!Activity = Val(Mid(Drow!Projectnr, 12))
                Drow!Projectnr = Mid(Drow!Projectnr, 1, 10)                
            End If
        Next
 
        Try
            mySqlDataAdapter.Update(DsTemp, "Humans")
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try

But in the end the Projectnr column has been updated with the Mid(Drow!Projectnr, 1, 10) but..
There is no column with the name Activity????
Does anyone know what I'm doing wrong here?