sqlbulkcopy error

Hi,
would you please help me out with this problem, i already stuck for couple days, any help will be greatly appreciate...

i'm working on a webpage with a temp datatable (one row and 30 columns), i try use sqlbulkcopy to insert to my sql table but with the following error:
System.InvalidOperationException: The given value of type String from the data source cannot be converted to type nvarchar of the specified target column. ---> System.InvalidOperationException: String or binary data would be truncated. at System.Data.SqlClient.SqlBulkCopy.ConvertValue(Object value, _SqlMetaData metadata, Boolean isNull, Boolean& isSqlType, Boolean& coercedToDataFeed) --- End of inner exception stack trace --- at System.Data.SqlClient.SqlBulkCopy.ConvertValue(Object value, _SqlMetaData metadata, Boolean isNull, Boolean& isSqlType, Boolean& coercedToDataFeed) at System.Data.SqlClient.SqlBulkCopy.ReadWriteColumnValueAsync(Int32 col) at System.Data.SqlClient.SqlBulkCopy.CopyColumnsAsync(Int32 col, TaskCompletionSource`1 source) at System.Data.SqlClient.SqlBulkCopy.CopyRowsAsync(Int32 rowsSoFar, Int32 totalRows, CancellationToken cts, TaskCompletionSource`1 source) at System.Data.SqlClient.SqlBulkCopy.CopyBatchesAsyncContinued(BulkCopySimpleResultSet internalResults, String updateBulkCommandText, CancellationToken cts, TaskCompletionSource`1 source) at System.Data.SqlClient.SqlBulkCopy.CopyBatchesAsync(BulkCopySimpleResultSet internalResults, String updateBulkCommandText, CancellationToken cts, TaskCompletionSource`1 source) at System.Data.SqlClient.SqlBulkCopy.WriteToServerInternalRestContinuedAsync(BulkCopySimpleResultSet internalResults, CancellationToken cts, TaskCompletionSource`1 source) at System.Data.SqlClient.SqlBulkCopy.WriteToServerInternalRestAsync(CancellationToken cts, TaskCompletionSource`1 source) at System.Data.SqlClient.SqlBulkCopy.WriteToServerInternalAsync(CancellationToken ctoken) at System.Data.SqlClient.SqlBulkCopy.WriteRowSourceToServerAsync(Int32 columnCount, CancellationToken ctoken) at System.Data.SqlClient.SqlBulkCopy.WriteToServer(DataTable table, DataRowState rowState) at System.Data.SqlClient.SqlBulkCopy.WriteToServer(DataTable table) at Embroidery_embroideryOrder.setTrailNum(DataTable dt) in C:\temp\embroideryOrder.aspx.vb:line 1438

here's my code:
                  'create datatable
                  Dim dt As DataTable = New DataTable
                  dt.Columns.Add(New DataColumn("c1", GetType(String)))
                  dt.Columns.Add(New DataColumn("c2", GetType(String)))
                  dt.Columns.Add(New DataColumn("c3", GetType(String)))
                  dt.Columns.Add(New DataColumn("c4", GetType(Integer)))
                  dt.Columns.Add(New DataColumn("c5", GetType(Integer)))
                  ...
                  dt.Columns.Add(New DataColumn("c28", GetType(String)))
                  dt.Columns.Add(New DataColumn("c30", GetType(Decimal)))
                  
                  'assign values
                  dim row = dt.NewRow()
                  row("c1") = "simon"
                  row("c2") = "crew"
                  row("c3") = "1234"
                  row("c4") = 2
                  row("c5") = 5
                  ...
                  row("28") = "VC"
                  row("c30") = 3.40
                  dt.Rows.Add(row)
                  
                  'display without issue
                  GridView2.DataSource = dt
                  GridView2.DataBind()
                  GridView2.Visible = True
                  
                  'insert to sql
                  Dim con2 As String = ConfigurationManager.ConnectionStrings("TestCon").ConnectionString
                  Dim copy As SqlBulkCopy
                  copy = New SqlBulkCopy(con2, SqlBulkCopyOptions.KeepIdentity)
                  Using copy
                        copy.BatchSize = 25
                        copy.DestinationTableName = "dbo.embTest"
                        copy.ColumnMappings.Add("c1", "c1")
                        copy.ColumnMappings.Add("c2", "c2")
                        copy.ColumnMappings.Add("c3", "c3")
                        copy.ColumnMappings.Add("c4", "c4")
                        copy.ColumnMappings.Add("c5", "c5")
                        ...
                        copy.ColumnMappings.Add("c28", "c28")
                        copy.ColumnMappings.Add("c30", "c30")
                        Try
                              copy.WriteToServer(dt)
                        Catch ex As Exception
                              Label6.Text = ex.ToString
                              Label6.Visible = True
                        Finally
                              copy.Close()
                        End Try
                  End Using
                  
here's my sql table definition:
CREATE TABLE [dbo].[embTest](
      [id] [numeric](18, 0) IDENTITY(1,1) NOT NULL,
      [c1] [nvarchar](17) NULL,
      [c2] [nvarchar](20) NULL,
      [c3] [nvarchar](100) NULL,
      [c4] [int] NULL,
      [c5] [int] NULL,
      ...
      [c29] [smalldatetime] NULL,
      [c30] [decimal] (10,4) NULL
 CONSTRAINT [PK_embTest] PRIMARY KEY CLUSTERED
(
      [id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

thank you.