How to make a message box pop up in user form

Ok so I put the code below but I get an error and when I debug it outlines the Sub field

Sub Update_Form()

Dim t As Control, res As VbMsgBoxResult

For Each t In Me.Controls
    If TypeName(t) = "TextBox" Then ' Make sure we're only looking at textboxes.
        If t.Text = vbNullString And t.Tag <> vbNullString Then
            res = MsgBox("You've not completed the " + t.Tag + " field. Would you like to complete it now?", vbYesNo + vbQuestion)

            If res = vbYes Then Exit Sub
    End If
Next
txtass.Value = Cells(Currentrow, 1).Value
TxtComp.Value = Cells(Currentrow, 2).Value
TxtDoc.Value = Cells(Currentrow, 3).Value
Txtdlname.Value = Cells(Currentrow, 4).Value
txtdoct.Value = Cells(Currentrow, 5).Value
Txtdocto.Value = Cells(Currentrow, 6).Value
TxtFname.Value = Cells(Currentrow, 7).Value
TxtLname.Value = Cells(Currentrow, 8).Value
TxtIns.Value = Cells(Currentrow, 9).Value
Txteml.Value = Cells(Currentrow, 10).Value
TxtPho.Value = Cells(Currentrow, 11).Value
Txtcellp.Value = Cells(Currentrow, 12).Value
Txtgndr.Value = Cells(Currentrow, 13).Value
Txtrac.Value = Cells(Currentrow, 14).Value
TxtDOB.Value = Cells(Currentrow, 15).Value
Txtage.Value = Cells(Currentrow, 16).Value
Txthin.Value = Cells(Currentrow, 17).Value
Txtwip.Value = Cells(Currentrow, 18).Value
Txtwd.Value = Cells(Currentrow, 19).Value
Txttcl.Value = Cells(Currentrow, 20).Value
Txttcld.Value = Cells(Currentrow, 21).Value
Txthdl.Value = Cells(Currentrow, 22).Value
Txtldl.Value = Cells(Currentrow, 23).Value
Txtbps.Value = Cells(Currentrow, 24).Value
Txtbpd.Value = Cells(Currentrow, 25).Value
Txtbptd.Value = Cells(Currentrow, 26).Value
Txtla1c.Value = Cells(Currentrow, 27).Value
Txtla1cd.Value = Cells(Currentrow, 28).Value
Txtedwd.Value = Cells(Currentrow, 29).Value
Txtdd.Value = Cells(Currentrow, 30).Value
Txtsw.Value = Cells(Currentrow, 31).Value
Txtevr.Value = Cells(Currentrow, 32).Value
Txtdn.Value = Cells(Currentrow, 33).Value

   End Sub